My mortgage servicer, Wells Fargo, did not accept a periodic payment from me as it was short a few dollars (they had re-figured escrow due, I had payments on auto pay from bank, and missed their recalculations), no notice from WFC, I only noticed because my bank balance was more than it should have been later in the month. I called them and found out the problem - of course even tho 99% of the amount due was in their hands on the due date, and the escrow balance was considerably positive, they charged me a 10% late fee. Sharp practice at best. Slimy banker practice at the human level.
Since there was no notice, you could probably get it reversed, eventually through small claims if they balked. But is it worth it. At least they were not being OVERLY punitive.