I make a lot of my own materials, and though I check things, I am human and make mistakes on keys and answer banks and such. Now I'm not saying that it's 50 bazillion mistakes on every assignment, but still.
I've just realized this year that that could be thought of as a good thing. My kids now know to be skeptical if their answer doesn't match mine. I hope that lots of them stop and think and redo their work and then in their mind if they can convince themselves that they have done no wrong, then they ask me to check things. This way: they gain confidence in their abilities, they learn to trust themselves, they don't always take the printed word at face value, they practice polite conversation in the how they approach me. Sometimes I'm NOT wrong even if they've done all this, but I use this as an opportunity to thank them for checking and to say that THIS TIME I was not wrong.
In a related type of situation, at various times when I ask questions of kids in class, and they answer, and the answer is correct, I put on a scrunched, disbelieving face and ask if they're sure or if that seems like the right answer. I've done this often enough that they stop and redo their thinking in their heads, and MOST times they go against the "ADULT" voice and say YES, that is the correct answer. Again hopefully they're being trained to stand up for themselves and their thinking.
Okay, ending with a silly joke that made all of us laugh on a much needed stressful day last week:
Q: What's brown and sticky?
A: A stick.