Beta stuff is stuff that was going to be in a game but was removed. Tales of Monkey Island has a few beta stuff such as Guybrush & Elaine dancing outside of pirate court. Bttf also has a few of these in the previews such as Biff & Marty talking outside the city walls in episode 3. A series of games i love has loads of beta stuff and the series is Kingdom Hearts. In Kingdom Hearts 1, Pride Lands was going to be in it but they could not work out how Sora would walk on 4 legs & it got moved to Kingdom Hearts 2. Disney Castle was ment to be playable in it too but was removed and put into Kingdom Hearts 2. Just look up videos on youtube for beta stuff. Also another thing in Bttf was George & Lorraine was inside their house in ep 3. If you want to know why i keep saying beta stuff in Bttf ep 3 it was my favorite Bttf the game episode ;).
Edit:If anyone is wondering where i seen the Kingdom Hearts beta stuff i saw it on a video on youtube & i read about beta stuff in the Kingdom Hearts wiki.