How many expansions and boosters are too many to use at once?

[Mister Ed]

Originally Posted by Munchkin

When I purchased my buddies expansion packs I also got another set of the main game. Would it make sense to shuffle that in if we mixed a bunch of these together or would that be a bad idea? 2 main decks mixed together with various expansions?

I know there are certain cards that can cause game-breaking problems if anybody were to get a hold of two copies at once, but I can't recall if any of them are in the base set.