Scams at gas stations

We've been in Mexico for two weeks had two and have two attempts made by attendants to short change us. A Shell station on 57 way south of SLP took the 500 peso note, hid it and held up a 100 peso note. I dug in my heels and adamantly refused to give the guy more having been suckered 5 years ago outside of Acapulco. Now I make it a point to snap the bill, show it to the wife, and am sure of what I gave. He finally relented and gave me the proper change. Then, just west of Cancun I had another attempt - which may have been partially successful - which involved giving me a really old 100 peso note as change. Like I said they might have gotten something over on me - but I insisted on speaking with the boss - of course by that time the old 100 peso note changed into a good one before we got to the manager....
In both cases two attendants were involved. I think that in the future I'll try to run the second guy off before the scam gets too confusing.
I've driven/ridden in Mexico 20 years or 30,000 miles and this is unusual.