Well, I can understand her anger over this. I was in her place at one time, and one step more...as he told me he also would cover the check and when he didn't...I got arrested. I was NOT pleased. Luckily, my lawyer was right across the street from the courthouse, the cop who came to arrest me allowed me to drive my own car down and park in lawyers parking lot and make a quick dash in to tell him what had happened. He had a paper bond delievered to the courthouse, before I was booked or anything else. But...mad...I could have spit nails. LOL (STBXH did end up paying all fines and the check...just trying to get me into trouble and it backfired on him. He ended up looking like an @$$, as this check business had already come up in court and he claimed at that time to have already taken care of it.)
In your case it may have been an oversight, but it's a bad one. Make ammends over this immediately, do whatever you have to do to make sure the bank knows this was not your W's fault, but a misunderstanding between the two of you. Write a letter if the bank keeps any type of file on this so they can put it in place. Also, contact whoever accepted this check and let them know that this was your fault, not W's.
When you've done everything you can do to make it right...call her up, tell her exactly what you've done and how sorry you are that this happened. That's all you can do.
Actually, it's a small bump in the road. It doesn't take away everything that you've been doing, and it will also be a chance to show her that you'll go the extra mile in correcting a mistake that you made. Eventually, that will count...but not today.
But she has a right to be angry, allow her to express her feelings. You don't have to be a doormat, but validate them.