my biggest mistake when choosing my backpack

we are humans, we made errors.
I start a topic about what was your biggest mistake when choosing your backpack.

I beggin : i live in a town where there is only decathlon for backpack choice. My biggest error : don't understand that a backpack is like shoes : you have to try and test a lot before finding the one you are the more confortable with, whatever the weight of your gears. And you? your biggest mistake?