Obviously this is about CDs/LPs/Cassettes that you regret parting ways with (whether or not it was your choice), yet haven't been bothered to actually replace for some odd reason. Here's some that come to mind out of my collection:

Black Sabbath- Paranoid
The first CD I ever bought, back when I was probably 11 or 12 and really into metal stuff (Megadeth, solo Ozzy, etc). In my mid-teen's, after being more into punk or whatever I gave it to my old drummer who was more into metal. Suffice to say, now that I'm 25 and appreciate a wider variety of music (and a newfound love for 'Doom Metal') I really regret this. I plan to get the box set someday though, to make up for it.

Misfits- Collection I & II
Had these for a few months in Highschool, let some kid borrow them, and supposedly his car was broken into and they were stolen. I always figured the bastard would buy them for me, or give me money, but nope. Ah well... maybe I'll buy THEIR box set too someday.

Screeching Weasel- multiple albums.
I had a pretty decent collection of this Chicago pop-punk band's albums, but sold them in the latter days of my jobless highschool years so I could re-string my Rickenbacker 4001. At this point, I'm not sure if I'd want to actually buy them all again, but sometimes I wouldn't mind hearing "Supermarket Fantasy" for nostalgia's sake.

Well, that's enough for now. Overall, I really try and make a point not to get rid of any CDs because let me tell ya... no matter how many discs you collect and how much you think you'll never want to hear something again, you usually WILL reach for it someday. These days it's rare for me to even let people borrow stuff; I used to be pretty laid back since I have 1000+ CDs and would figure "Well, give it back whenever, it's not like I was going to listen to it tomorrow..." This just leads to laziness and more lost discs. Anyways, feel free to share your stories/regrets.

The Superfly soundtrack with Curtis Mayfield.

Concert for the People of Kampuchea

I traded it for something else in '84 or 85. Bad decision.

I've had really good luck in that I haven't had very many CDs stolen from me, but several years ago, I did notice that about a dozen of them had gone missing. Some, I didn't know were gone until months later, when I went to play them and they weren't there. I think my (now) brother-in-law did it, but I have no way of proving it.

Beck - Odelay - Replaced couple of years later
Hole - Live Through This - Never replaced
Mad Season - Above - Replaced when Layne died, and I went on a big A.I.C. kick
Metallica - Ride the Lightning - Replaced a couple of years ago, when I went on a big old school Metallica kick
Metallica - Master of Puppets - Same
N.W.A. - Straight Outta Compton - Replaced a couple of years ago when a remaster came out
Tom Petty and the Heartbreakers - Greatest Hits - Never replaced
Prince - The Gold Experience - Never replaced, but more because it went OOP and I couldn't easily track it down
Various artists - The Beavis and Butt-head Experience - Never replaced

I think there were a few more, but that's all I can think of right now. I've got a list somewhere.

Then there's the CDs that are loaned out to people and never returned, for whatever reason:

Joni Mitchell - Blue - Replaced several years ago with the remaster

And I've done it to people, too:

k.d. lang - Ingenue
Tesla - The Great Radio Controversy
Soundtrack - The Doors

Not that I intended to keep them, but sometimes you just lose contact with people.

Led Zeppelin - Led Zeppelin II: "Borrowed" by a guy in high school, I never saw it again. Later he tried to insinuate that I stole his Iron Maiden "Live After Death" CD but my copy was courtesy of a used CD bin from college. Replaced with a new CD.

Danzig - Danzig I: Lost, not replaced (yet).
D.R.I. - Thrash Zone: Lost, not replaced (yet).
Primus - Sailing The Seas Of Cheese: Lost, replaced with a used CD.

I'm usually good at maintaining my CD collection but somehow I managed to lose the above CDs (still have the jewel cases though). I think they may have been in a Pioneer six-disc CD changer that was involved in a flooded basement at my parent's house.

My cousin "borrowed" my GN'R Illusion II - 6 months ago and I finally replaced it for $2 at Flea Market

Coroner/No more color-Grew out of my thrash phase in the early 90's and gave away most of my metal cds. Right now, I'm missing this one. Forgot how pumped up I would get when "Die by my hand" would play.
Geto Boys/We can't be stopped-Stolen out of my walkman at school. The album wasn't really all that, but I was saddened to not hear "Mind playing tricks on me" for awhile. Got a compilation CD a year later which had the track.

My college roommate "stole" Led Zeppelin's "Physical Graffiti". He denied it, but I knew he was lying. He also cheated me out of his share of the phone bill. Bastard I did replace it later.

I lost Springsteen's "Live 1975-85" set (the whole thing) Have no idea how I could've done that, since I loved that set and played it all the time. I have a copy from a friend, but still haven't gotten around to rebuying an original for myself.

I lost (or had stolen) Genesis' "Three Sides Live" ... ironically enough while camping out for a Genesis concert in the mid 80's (during the "Invisible Touch" tour, for a show in Philly). Still haven't gotten around to replacing it. Too bad ... I really liked some of the live renditions on that, especially "Abacab" and the "In this Cage/Afterglow" medley.

I let somebody borrow Ministry Psalm 69 and I never got it back and that was like 5 years ago.

I also lost Rush's Different Stages disk 3. I was working on serveral PC's at the time and I put it in one to make sure the sound card worked and I forgot to take it out.

Well these are neither lost nor stolen, but they were more or less ruined. Both had water/beer spilled on them:

Guns & Roses "Appitite For Destruction" (original LP album cover; an ex-roommate poured a glass of water over it, ruining the vinyl and my turntable)
Various: "Hard to Belive: A KISS Covers Compilation" LP (some drunk idiot spilled a beer on it when I was DJing).

Both were/are totally rare.

In the 70s I had TONS of kids' records, including just about every Sesame Street record. In 1982 I decided I was too old for them, and one of our friends was having a garage sale so I sold them all there. I don't even remember what I did with the money I got from them, but I want them back now, dammit! That's why now I NEVER get rid of ANYTHING.

Around 1987 I came very close to trading in my original West Germany- printed CDs, complete with longboxes and inner trays of Madonna's Like A Virgin (the very first CD I ever bought, in 1985) and Van Halen's 1984, at the first used CD shop to open in town since I had become ashamed of them. I ended up keeping them because the store didn't have anything else that I wanted, and now I'm glad I still have them.

Radiohead - The Bends: WTF was I thinkin'?

I've had very few cd's go missing. Especially these days, now that I make a copy of them to take with me places. That, and I have friends that aren't going to steal from me. I haven't traded out many albums, mainly because I'm too lazy to get rid of the few albums I don't like anymore.

Anyway, the only cd's I've had stolen that I was upset about were:

Def Leppard - Hysteria Definately one of my favorite albums, at the time. I ended up buying this again from a used cd store.
Dave Ralph - Tranceport 2 - First cd of a two cd set of trance music. I've never replaced it, mainly because the 2nd disc is better.

I had all 300 albums stolen about 20 years ago. It still annoys me.

I've replaced most of them (mostly with CDs, although a few had to be replaced with records). Some (like Suburban Lawns) were just a bear to replace.

There are two Rodney on the Roq disks I haven't been able to replace yet (I've tried to purchase them on EBay, but somehow I never get charged and they never show), and various other LA/California stuff from when I was in high school--that stuff is sort of hard to replace (I have to admit, in some cases it is hard to even remember but I'd still love to replace it). Also the albums I got when I was an exchange student in Germany--I've tried but not succeeded on some of those.

I had a girlfriend who began experimenting with drugs (eventually becoming a crackhead after I tossed her out) while I was at work each day (I often work 12-14 hour shifts). I came to find that, when she wasn't raiding my wallet or writing checks, she was raiding my CD and DVD collection.
One of my favorite CDs was sold by her to a local used CD store for about $8, a copy of The White Album SIGNED BY Ringo Starr. I obtained the autograph myself, and it was an all-day game of cat-and-mouse between Ringo and us fans. Cool item for me; a $5 rock for her.

Bitch.

i used to have a job that i could listen to headphones while i worked. so i took a 60 disc case with me of my fav. cds. well i got drunk one night after work and lost the case walking to my house. that really sucked.

I had the original Dr. Octagon indie label tape which had three different songs from the Dr. Octogonocolygist cd version which is the only official in print version. I let someone borrow it or something and never saw it again. Grrr.

When I was 24 in '89 I had just begun to seriously feed my CD habit. Being cheap & poor I started to trade in some old classic rock albums that I knew I'd never want to hear again (this was at the heighth of my all things Replacements phase). I didn't regret this decision for many years, but recently I've wanted to hear some minor rock bands that were local to Indianapolis (Roadmaster, Faith Band). It's mostly a nostalgia thing and I'm not serious enough to purchase them on Ebay.

.... goes and checks Ebay.

I wish I had the Star Wars original soundtrack LP.

I remember buying it when it came out in the late '70 and I hadn't seen the movie yet. When my family visit the Bay Area we saw it at the Century 21 theater in San Jose. After seeing it in a packed crowd, it became my favorite movie of that year. Ahhh...those were the good old day.