I can sympathise with warwickhunt. Had a gig on Guy Fawkes night last year, was called up on the Wednesday by a lead guitarist I get on with, asking if I was doing anything on the Friday night. Turned out his covers band had bailed out last minute and was scraping together other musicians. I hesitated, he said 'don't worry it'll be fine'.
Had to take the Friday afternoon off just to brush up on 40 songs (!) when I got there, I discovered he plucked up a drummer on the same day as gig! Best of all, the gig was at the Pickerel in Slowmarket. Now I know why his bandmates jumped ship last minute! Never play there people!
I am now in an original band with the said guitarist. Pulls the same stunt still but have learnt to say 'no'!
I know every situation is different but some last minute things are never meant to be...
Luke