What’s one thing you wish you’d known before you started testing?
For me, I wish I’d known that it wasn’t going to be horrible 
I remember being so frustrated when it was decided that I should move from dev to testing thinking that it was some sort of a demotion. I now, of course, know that isn’t the case but I wish I’d known it then.
Years before I started testing, I would have wanted to know that testing was a viable career choice.
If I went back to my first testing job, I would want to know how powerful automated checks could be. We wasted an absurd amount of time retesting things which should have been automated.
(non-functional) Requirements 
So much this. I was late to the game, 37 years old, when I found out this was even an option. One of the reasons I went back to my old school to talk about it.