Out of about 700, I have three. All three are part of TV sets so it's not so easy to just toss and rebuy.

Quantum Leap: S3 disc 1 (the wonderful Universal DVD18, totally unplayable)
Northern Exposure: S2 disc 1 (got two disc twos instead of disc one)
Heroes: HDDVD: S1 disc 5 (had a scratch on day one and skips during one ep.)

Out of around 580+, I've only had around 3.

*Alias: Season 4 (Disc 3 would skip randomly during playback)
*Brotherhood of The Wolf CE (Disc 1 of 3 missing)
*Opera (2 Disc AB Edition, Disc 1 would not play)

Over 1200 and the only defective ones I have ever had were ones known to have problems like some of the early Anchor Bay ones that I already replaced. I have replayed many discs from 1998 and have yet to find a bad one.

Two.

Disc 2 of Season 2 of Smallville (I actually exchanged this one twice and all were the same.)
Disc 1 of Season 4 of Angel

It annoys me to no end knowing that they're in there.

Using the disc as a coaster and the case for a working disc would be more 'serviceable.'

With over 2300 DVDs, I've been lucky to have had less than 10 overall that were defective. At least, 10 I know of since I haven't watched every title I own yet. The most frequent offenders lately have been the few HD/DVD combo discs I have. Just last night Eastern Promises fucked up again, for the last time. I'm going to return it and buy the SD version and wait for a BR version to hit.

I have 2 bad discs from 2 sets. But they are just the cheap Mill Creek Entertainment sets you find at the Wal-Mart dumpbin. One disc, a bunch of TV shows, has one episode freeze up several times while its playing. The other disc, a bunch of cartoons, has the player freeze up and make noises after the 25th cartoon on the disc. The TV show disc I will keep and just not play the one episode that freezes. I bought another copy of the cartoon set to replace the bad disc from that set, but have to keep both copies of the set because they replaced a few cartoons on the new copy of the set.

I've only encountered three and they happened to be imports a Peter Greenaway Collection set disc from France, Destricted from the UK, and a copy of My Name is Joe (UK again)

I've had a couple messed up discs. The first was State Property. It began pixelating near the end until it finally wound up freezing completely. The other I just discovered today was The Sixth Sense. About an hour and 15 minutes in, it began kind of "hiccuping" for a second every 30 seconds or so. As the movie progressed, it got worse.

Same thing happened to me. It worked fine when I first got it, but I went to play it a while back and it was, well broke. The irony was not lost on me. It's the only disc that I know of that broke after I got it.

when i first bought under siege it would not play (it was a lil ruffed up) even tho it was new, but being a flipper these things happen i guess

I have over 4000 dvds. The only 2 are the first releases of THE THING (Kurt Russell) and Appollo 13 (Tom Hanks). At first, MISSION TO MARS was messing up but worked in a different dvd player.

I received two consecutive bad copies of "Of Mice and Men" from Blockbuster. The third copy played fine, thankfully.

Been lucky so far.