You flip the table upside down, yell at every player (and at the whole world by the way), chase them out of your home, and make a vow never to play with them anymore
Just kidding !
Such mistakes may happen. If the turn during wich the mistake has been made is finished, it's too complicated to correct it. Just go on playing, keep having fun and laughing
It's just a human mistake of the player who placed the tile, as well as of every other players who didn't notice it. This may happen to any of us.
As long as it isn't a habit to try placing wrong tiles without others noticing it it's not a problem. And if a player starts having such habit, well... Play with someone else
(or make a statement at the beginning of the game that any player caught making such placement will get a penalty (-20 points for example)