I once let a shop flush my antifreeze (i know, i know). Next day, coolant hissed and seeped out of the reservoir cap, as if it was overheating. I bought a new cap; did the same thing. I called the dealer back and the tech said that sometimes happens when the reservoir cap is over-torqued - the plastic threads aren't strong enough. I backed-off the cap about an 1/8 of a turn and it was fine...for about a year, when another tech removed & retightened it during the free "courtesy check" that tire shops like to do (because they're not making enuf money off you already??). After that, I used a sharpie to make an alignment mark on the cap & resevoir.
"I'm jealous of girls because they get to wear dresses." (Todd Flanders)