I did it about 10 years ago at a busy garage on the A40 into London. I was convinced someone must have pinched it and as it was only £20, I didn’t bother calling my bank as I felt it was just my bad. It was shown as gone from my account so I just put it down as a life lesson.
About 2 years ago, I received a letter from my bank saying that they had done an audit and it had come to light that this happened and the machine did take it back in, but it wasn’t credited to my account, so they gave me back £20 plus 8 years interest!
Make sure you call them and get it back now.