Sort of a related subject:

When a game progresses so rapidly that you get left behind because you can only check it once a day.
As much as people bitch about others never posting, can we also acknowledge how bullshit it is to have to read 3+ pages of IC back-and-forth because you spent a day at the zoo instead of glued to your laptop? 

I actually got killed in the first GM I played, (It was a really shitty game, actually. The GM didn't really know what he was doing, nobody else would join, and I kept Godmodding, and he wouldn't stop me. Aah, good times.) because there was a chain of me and the GM rapid posting after another, and then me leaving for a few hours. When I got back, the GM had gotten bored and frustrated since we were in a tense scene, so he flat out killed my character.

There's no real way around it in forum based RPs. You can try and force turn-order on people, but that will restrict the natural flow of the game.