I've noticed alot of alterations being mentioned on television series DVD's at tvshowsondvd.com.

My question is how are people able to remember what the original broadcast version of an episode was. Especially from the ones that aired in the 70's and 80's. It's amazing that someone can remember that a certain dialogue or scene had been cut out.

I can see if they happen to have a video tape of an episode and compare, but this scenario would be highly unlikely for episodes from the 70's when VCR's were not commonplace.

I also have wondered about this, but how many shows pre-VCR days do we hear about alterations? (This is not rhetorical...I am really wondering)

While not commonplace, they were around. (I got one in 1977 and I'm quite sure they were available before that.) And videotapes can be copied and widely circulated.

The people noticing these alterations (edited scenes, replaced music) are usually fans who have been watching the episodes over and over for years/decades. I had a set of Battlestar Galactica episodes from the original broadcasts on VHS prior to the DVD set coming out, for example.

Think of your favorite film that you've seen over and over and over again. And then imagine watching it with stuff missing or changed. It would leap right out at you. (I could tell you every single change to the Star Wars trilogy -- even every trivial audio alteration -- just by watching the SE once. I knew those movies backwards and forwards.)

That's what's happening here... and why some people are up in arms while others might not care so much.

http://en.wikipedia.org/wiki/Video_Cassette_Recording

I also recall Cracked magazine back in 78 or so was giving away VCRs as a prize in a contest for their readers.

Man, I feel dated now.

Well the main answer to this question is, of course, kinescopes:
http://en.wikipedia.org/wiki/Kinescope

There is another answer as well:
Believe it or not, in the 60's and early 70's (prior to the introduction of the home VCR machine) many people recorded the audio portion only of their favorite TV shows on reel-to-reel and cassette tape.

At one time I owned several reel-to-reel tapes of the audio portion of the original network broadcasts of various episodes of Batman and Maverick - and even the original Star Trek TV series.

In fact I know of many college people of the day that had their first exposure to the Star Trek episode "The City on the Edge of Forever" via a cassette of the audio portion of the show. I remember that one of them told me that after they had finally had an opportunity to actually view the episode in syndication that the "Guardian of Forever" didn't live up to his expectations of what he had imagined it to look like!

There are a lot of un-edited 16mm prints in the hands of collectors, some 35mm too.

I have several episodes of "Bosom Buddies" that I audiotaped before I owned a VCR. I would be able to tell the edits by comparing the audio tapes to the DVD episodes.

And yes, I recorded the episodes during the original ABC Network run...

People have also interned or worked at tv stations and made copies for home use. Good thing they did because some of the footage housed at the TV station either was thrown out, lost in the sale of the station, or locked away in someone's attic and forgotten. BBC tossed a bunch of footage out, including the first TV appearances of Pink Floyd with Syd Barrett.

Its the power of the Internet. With hundreds of millions of people all connected, each with their personal obsessions, all it takes is a handful of people, or sometimes even just one person, to catch an error and then we can all know about it. Information wants to be free, and this is just another example of how the net sets it loose.