Okay so i found out a duplication glitch with hopper minecarts!
Look at the pictures below, please patch this dupe bug.

you make a an L shaped rail put 5 hoppercarts together on same rail peice
then throw any item in to the hopper by pressing Q, throw 3 in the hopper.
now you punch it, fake (item used) comes out, after you break it a ton of (item used)
drops.

this happens in survival & creative.

video proof of the duplication glitch

http://www.youtube.com/watch?v=RBseHC2v2b4&list=UUQwB4yvdk0Ka85lMD5iMpUg&index=1