So here's a funny story (well, maybe not).
About 15 or 20 years ago I owned a Bultaco 250MX. I bought it as a teenager, from a local farmer who used it to herd sheep or something I guess. It actually looked in pretty decent condition, and being a naive young kid I never did anything much to service it. Anyway, it had this annoying habit of the clutch slipping like crazy for about the first 10-15 minutes of riding, after which time it would finally 'grab'. I never bothered investigating..
I sold it to another local kid a couple of years later. One day he comes up to me and says "Did the clutch ever act funny?" I say, "Why do you ask?". He says, "Well, the transmission was mostly dry except for a few ounces of dirty water..!"
;)
-pace