There are several instances which, to me, make for the worst, most game-destroying bugs. They only occur in multiplayer, and I believe there are two varieties; the creature identity bug and the different representations bug.
The creature identity bug results in multiple creature representations being shifted a column. For instance, an opponent's dragons will move 1 spot to the right, into a devourer spot, while that location retains its status (burrowed). This is often associated with the 0/0/inedible status.
The other bug, which is worse because it occurs more often and its effects are more one-sided (and against my kind of deck), is the different representations bug. What I believe causes this bug is that the game states are separately stored (locally) for the two players, instead of a unified game state being stored on the server. As a result, sometimes changes made by one player do not affect the game state representation of the other player.
For instance, I play chaos seed on an enemy otyugh, resulting in it's health dropping from 6 to 1. I finally play my Elite otyugh, now that it's safe. However, my playing of the chaos seed never registers on my opponent's screen, where his Otyugh's health is still 6, and he thus proceeds to eat my Otyugh. On his turn, my game suddenly shows its health back on 6 (and then 7, as it eats mine). The same thing can occur with lightning not taking effect, or even with devour not taking effect. I have experienced several cases where I devoured a mind-flayer, only to have it re-appear and lobotomize the otyugh on the next turn.
At the risk of being redundant: this SUCKS! A fix would be MOST welcome.