I learned that not all the similar-sized lids fit quite right on all the similar-sized jars I have.
I have a habit of removing the labels off every sauce and pickle jar I use. I always seem to need them to store sauces I make, as well as things I'm pickling and fermenting. Most of the lids are close enough to the same size. As I reuse containers, it's not always clear which lid went to which container, and it usually doesn't seem to matter.
I suppose it's more accurate to say I was reminded of this fact. I was shaking the contents of jars I was using to make gin, and some of the liquid splooshed through the lid of what I thought was a closed and sealed container. Oops.
Sometimes, I need to be reminded of things I already know.