I had this happen before too, it must be an undocumented bug. I had my thief drink potions of master thievery until he was maxed, and he would fail every time. I rested off the potions' effects and then tried again, this time stopping just below maxium. You don't really need more than 200 for any merchant, but there will always be a 1% chance of failure.
------------------
"This place will be your tomb."