Ugh, that's the worst! I learned the same lesson the hard way with a tile job. My rule now is to check with a 4-foot level AND a marble. Roll the marble around, if it takes off, you know you've got a slope. That double-check has saved my butt more than once.

Honestly, I've had the opposite happen. My good scanner caught a weird misfire code that just looked like a rough idle. If I'd just checked the cap I'd have missed the failing coil pack. Sometimes you need the tool to point you past the obvious stuff, you know?

Yeah mirrors can backfire like chen.jade said, gotta watch for that.