Small deeds, small lessons: what is one minor thing that you learned never to repeat?

What small but valuable lesson have you learned from a single memorable experience?

I’m not talking about big or even biggish things like never buying a car from a friend, or not planning anything else for the day you had the lower GI xrays, or not flirting with your sister’s boyfriend.

More like these:
•  Never having a spicy meal the night before a job interview.
•  Not sticking a Coke in the freezer to chill it quickly, and then forgetting about it.
•  Never trying to make a burrito with creamed corn, which is what I did last night.

What’s something you had to do only once to learn your lesson?

(“It seemed like a good idea at the time.”)