I am a bit embarrassed that I sometimes get my "there" and "their" mixed up and occasionally miss an apostrophe or add one incorrectly.
I think I understand all the rules for these sort of common grammar mistakes, but can fail to apply them if I am not taking sufficient care.
"You can't write proper English under pressure" is a site which puts this to the test.
It's quite fun - have a go!
Though it auto starts music which is annoying - especially if you are at work, so hit mute before clicking!!