I have never had this happen with an SP but it has happened about 10 times over the past 25 years where I tipped extra $$ at the end of a session (I usually pay up front for everything) because it was such a good session and the ladies refused the extra money.
I have also had a couple of occassions where I was seeing my regular MP and we both forgot the room fee and the tip. I was a couple of blocks away when I realized I had forgotten to pay. I immediately call her back on my cell phone and said guess what - I forgot to pay you! She laughed and said she totally forgot. I turned around and drove back and paid her.
I have had occassions where I was seeing an attendant for the first time. When we discussed service level they would quote at least $40 more than the typical going rate. I would either lower my expected SL or pay it, but I would never see that particular attendant again.
I've even had the nickel'n dime approach used (although not for many years) where the attendant keeps trying to raise the SL from say a topless to a nude, then nude reverse, then body slide, etc each time bumping things another $20 to $40.
Unfortunately I have also heard stories of guys who think that the going rate for a body slide is $40. Or the ones who think that a visit to an MP is like bargaining in the souk or flea market with the local street vendor. They want the world (and more) for $50 and some actually think they are doing the MPA some kind of favour by allowing the MPA the opportunity to experience a session with THEM
I've also been told by an attendant that if I wasn't interested in at least a body slide then I could pick someone else. This was after I had paid the house for the hour and already taken a shower. I said "Fine, send someone else in." The follow-on lady was very nice, didn't insist on any particular SL and we had a great time. At the end of the session she asked what had happened with the first girl and when I told her she said that the first attendant was a stuck up bitch who treated the rest of the staff like shit. I made her day by tipping her an extra $20 over our agreed rate.