So, no allowance for the DM simply not knowing something was an issue until play began? Perfect knowledge or you walk?
I'm starting to wonder what a lot of people here want from a DM. Because the DM-hate across the site is getting thick.
I suspect a very large % of the posters on this site DM - so would be surprised at actual DM hate!
But to answer the first sentence: If something proves to be a problem in play, you discuss it with the player AFTER the session and see what kind of accommodation can be reached. I've never seen banning something (especially something RAW) during a session lead to a good place.