Proofreading.

[sister_luck]

I think it's very annoying that one should remember to double-check all teaching materials for spelling mistakes. Especially if the material in question is provided for the use in tests and exams.

They have to invent something that makes live safer.

Yes, it's one of the classics and no, I didn't spot it right away either. I noticed it in my students' work and it took me a while to realize that it was spelled that way on the worksheet, too.

Why, yes, I'm deep in marking hell.