As a tiny test, did you immediately spot the typo I deliberately inserted in the above paragraph?

Hmph. I was seconds away from typing a snarky reply about it, but then you went and ruined my fun.

On a related note, I suspect that people are better at noticing typos in other people's writing compared to their own, possibly because they know what their own writing is supposed to say and have it somewhat memorized. However, this is unfounded speculation and generalizing from one example.