It's like a compulsion, really. I get a nervous twitch when I see anyone regularly swap “its” and “it's”. Of all the ones to confuse, how can you confuse those two, since you can easily sound out “it's” to “it is” to see if it makes sense?
I wish there was some way to see what the right answers are.
 The occasional indiscretion is to be forgiven, it could have been a typo.