I closed an account with Santander and they sent the remaining monies in it to another bank account.

Then they accepted a charge on the account from a subscription I had forgotten to move, even though the account was closed, and because my account was then in deficit decided to reactivate my account without letting me know. Then they charged me an overdraft fee and daily penalties, and then eventually sent me a bill with like £120 of charges.

I challenged it and had to speak on the phone to people for ages. Eventually they took the charges off as a "one-off gesture of good faith" which annoyed me, because it still implies that it was my mistake and not theirs.