I know that ticks you off in me, too - though I usually only reserve my expressed incredulity for people who can really, really be expected to know better - like uni students who don't know that we're indeed in a solar system. And even with those, I wouldn't express that sentiment in front of them rather than patiently explain.

So, er, in conclusion, I'd like cookies for keeping it together before cracking up about it out of earshot, plz. :D

Students' mistakes (especially students' mistakes at a tutoring centre) are interesting rather than funny, though. It's also a way to see what kind of thing falls through the cracks at school, because there are definitely patterns.