What do you mean by failed commission? Having to cancel a work in progress or having to decline a piece?
I do a lot of character commissions and in the early days I ran into a few issues here and there. Most of them were resolved by having a thorough terms of service listing and pricing. As far as cancelling a work in progress I've never experienced that but I have cut ties with clients over the years for being extremely picky or requesting a multitude of changes that weren't stated with the initial request. It's tough sometimes but its important to approach any issues with respect and a level head.