This was many moons ago but the rule still applies.

DON'T visit a Macdonalds where your ex-girlfriend/boyfriend works at.

A young man was unfortunate enough to come through the drive-thru and order a cup of tea. His ex-girlfriend (the shift manager, no less!) decided to make the tea herself but with an extra ingredient. She deposited a healthy gob of spit into the bottom of the cup before pouring the tea. She made sure she gave it a healthy stir so there was a minimum amount of 'stringy bits' then gave it to another female colleague to present to him.

It's quite simple, if someone is going to be serving you food or drink. Be nice!