One story that details said craziness is that of the last Pink Floyd song he rehearsed with the band, entitled "Have You Got It Yet?" What Barrett would do is play the song to the band until they had a pretty good understanding of the structure, then he would change the arrangement slightly to throw off his band mates, hence the title of the song. Roger Waters recalls the situation pretty well in this video:
Unfortunately, that song has been lost to time, so you'll have to settle for some of his other psychedelic antics.
Got any favorite crazy musicians?