"It works in the simulator..."

I do most hardware development in simulation. It allows a level of visibility that software developers can only dream of: I can see the state of every signal at every instant for the entire run. But there is a dark side to simulation: the synthesized design may not behave the same as the simulation. There are a few well known patterns that lead to synthesis/simulation mismatches that are easy enough to avoid with attentive coding, but the existence of these gremlins always leaves me uneasy...