Whether it's a messy situation, or not depends on when you decided to withdraw... most schools require you to accept their acceptance via signing a form, writing a letter etc. So if you sent a withdrawal letter without ever sending anything to affirm the acceptance you are probably okay. The school would have called you by now to see what was up if they never received any kind of response, as there are many people on waitlists everywhere dying to get in. However if you decided to withdraw AFTER acccepting a place in the class, then you may want some kind of acknowledgement, so you are not holding multiple acceptances after May 15th. I received emails and letters acknowledging withdrawals but as sorrento said they took over a month to arrive.