I love it when iChat blocks you from talking. You might have two coworkers going in separate windows, conducting actual work conversations, then you get blocked with this:
The AIM service could not send the message: You have sent too much data too quickly. Please wait a little while before sending more.
I call it the "Shut the fuck up for a second" feature. I know that real AIM clients used to have limits to keep you from crapflooding users, but iChat seems to invoke it under normal circumstances every few days for me (it helps if you're a fast typist).
I hate when software knows what is best for me, and I have no say in the matter.