It turns out that iChat doesn't deal with the AIM server's buddy lists so well. I have buddy groups, and whenever I do any buddy list editing in iChat, whoever I added or edited gets duplicated in all my groups. This doesn't show up in iChat. Everything looks fine, but if you open AIM and look at your buddy list, you'll see the same people repeated over, and over again. This happens if I add someone new, or if I add a buddy icon to someone already on my list, or do any changes at all, really. Since I have well over 100 buddies, and 6 groups, having several of them duplicated causes me to hit some sort of buddy limit. I think it's somewhere between 150 and 200.
So, every now and again I use AIM to delete all the duplicate entries in my buddy list, and everything seems to work fine.
The easy way to spot the dups is to start with a clean list (no dups) and use AIM to sort them. Then, the next time you go in to clean up the list, all the duplicate entries appear at the bottom of each group. In the group where the buddy belongs, the buddy is in the right place in the sort. So you can just delete all the ones, in each group, that come after the last correctly sorted alphabetical entry.

