Fwiw- I once made the same error. I still don’t know how it happened, bc I always tip (unless the provider was a scam artist- and even then I pay the regular fee). It was a small town in California and after the massage, I drove downtown for some lunch and the provider *followed me into town in his car* and approached me. Kinda made me nervous (he didn’t call me?), but then I was mortified when I discovered that indeed there was $40 extra dollars in my wallet.
Just sharing to say it can happen to anyone!