someone else had to have run into this problem as I did. during the 2nd encounter with the scarecrow and his maze, there is a part where you jump on a ledge (show at 5:30 in the youtube video with this title "Batman Arkham Asylum: scarecrow dream 2(wayne manor+parents death)")
now I was stuck at this part because you are supposed to be able to drop down from the ledge on the other side by pressing A, but for some reason you have to press B
This took me about an hour of searching on the internet and messing around with the game to find out, so I figured it would come up with someone else
Hope it helps!