1) When playing with the dragon, and a field has a farmer and a pig in it, then the dragon comes by and eats the farmer (leaving the pig).  The game will not let anyone farm in that field (even if it's the same person who owns the pig).

2) I've seen several occasions (not in the basic game but with some expansions) where some players will be given extra turns for no reason (their builder was not on the board). 

The first bug is annoying but fair (it effects everyone equally).  The second bug can be very unbalancing.  Ugh.