This one is actually not that easy to repeatedly test here - because it happens only sometimes. I am still guessing though that is has to do with the number of message windows you have open in the backgrounded skype. At least the two times I had the error, I opened skype to check the logs there to see where it went - and I had 4 or 5 open message windows with old messages I had sent before. I closed them all, restarted skype and adium and it worked again. I can't really reproduce that easily, because I only have 4 or 5 skype contacts in total that I chat with - and that is pretty sporadic ;)

The plugin sometimes sends messages to the wrong people, although I haven't been able to figure out why. The messages in the adium log are then to person x and in skype log to person y. Maybe it has something to do with the number of open message sessions in the backgrounded skype.