Question: What do you do with bugs?
Answer : Stop making them.
OK, that is facetious but in an XP/Craftsmanship environment you should be approaching zero bugs in production.
But - you still will get bugs. It's a fact of life. So how does FAST propose to handle them?
Well really I want you to work that out by creating your own process. But - here is an idea that you might want to run with or adapt :-
FPM looks at the marketplace and decides which is the best team to handle the bug and is least disruptive to the iteration. He walks into their room and tells them to fix it as a higher priority than their chosen work. They may or may not complete their work as a result. Perhaps they cut scope and did manage to complete or maybe the work/story will have to spill over.
The FPM puts the bug up on the Project Map if they want it fixed. Bugs take highest precedence to all story work and must be picked up by one of the teams at the start of the next iteration. Bugs mean you don't have working software - which is the primary measure of progress. Therefore you are not making any progress until the bugs are fixed.