I don’t want to jinx our good luck, but we have not had a bad vacation, just a few bad moments that did not ruin the trips completely.
Our first trip to Riviera Maya… Mr. decided that it would be fun to rent a car to be able to move around freely. I read up on it and was not so sure… there were stories about rogue cops, car rental agents demanding bribes upon return… lots of shady stuff. Well, what they said on the web was completely true! As we were pulling out of the hotel’s drive, a cop stopped Mr. “for speeding.” We were going 10 mph below the speed limit. I quickly told Mr. that the standard reply was “how much?” $150? Mr. pulled out his wallet, and luckily he only had 3 $20 bills in it. Apparently, that was enough. The cop quickly shoved the money into his pocket and went after another victim coming down the driveway… While we were looking for Costco in Cancun, an angry dude trew a rock at us from his car. Luckily, he was not that good at aiming, and the rock missed the windshield. Upon return of the car, the Hertz agent claimed that we lost the spare tire and needed a deposit of $1,000 because the car was not rentable without one. Or he would call the cops. Mr. showed him the photos and the notation on the rental contract that clearly showed there was no spare (again, I insisted on all of that because that’s what I read on the internet, lol). Plus, we wisely bought extra insurance… and we told the guy that we would pay with a credit card and then dispute the charge. A big mess averted. Needless to say, I will never rent a car in Mexico. 
Other than that, the vacation was great. Fabulous hotel and food, good beach and a great private tour of the ruins.