The old spellchecker gremlin is alive and well, getting student writers into trouble. I saw plenty of this as a professor, the student writes "there" when they mean "their" and the checker doesn't catch it because both are valid words.
Here is an article about this issue from The Telegraph. I'd see to, too, and two mixed up, and there are others. Since I taught Management, one of my favorites was when my students wrote "manger" when they meant "manager."
Both manger and manager are, of course, valid words meaning quite different things. I can't tell you how often I wrote "manger =/= manager" on a student paper.
(N.B.: =/= is as close as I can come here to writing the math symbol of an equal sign with a slash drawn through it, meaning "does not equal.")
