The highest online count was stuck on the 9th September 2006 for a while (the day my son was born), I guess that was a major event.
S.E. Spiders - its possible, but i think we did a mod that stopped spiders getting a session var set (and hence stopped them being counted in that count) cause it was hammering the crappy server too much.
My idea was that if there were say 30 people online and the server has a wee flacky, then all those people would get a new session variable and hence the number might doubled - but I'm not sure that argument works either.
Mibees we just had a few busy days cause its wet and there is alot of glasgow stuff in the news..