My sister, coming into my work and seeing me using my knife and telling me "Ummm, that's actually really dangerous! You'll cut yourself doing it like that."
So then, I explained to her that I've been using my knife exactly like that for the last six years and not once cut myself.
A couple of hours later, for the first time ever, I've sliced my thumb right open.
I remembered what my sister had said when she came in for a while as a customer earlier, then looked at my thumb (bleeding quite heavily) and just lol'd.