There are some typing mistakes I can't help but make. I type fairly quickly, and so my fingers tend to be on auto-pilot most of the time. I think the word, and my hands do the rest without much interference from my brain.
However, this can lead to problems.
I have a friend called Michaela who I've known for many years. I've typed her name so often - it was her username on a messaging system we used to use at University besides anything else - that M I C H A E L A has become almost reflex when I type it. This causes problems with friends I have called Michael. When I start "M I C ... " the auto-pilot takes over, and before I can stop myself I've typed "Michaela" and headed onto the next word. It's certainly not the most embarrassing example, either.
I use the word "bugger" quite a lot. It's one of my favourite expletives. I like it because it sounds a little less extreme than some of the harsher options (which I shalln't repeat here). However, because I type "bugger" quite a lot when I want to use an expletive, it causes issues whenever I want to type "buffer" which is far too close for comfort on the keyboard. In my work, buffering is quite a common concept (stop sniggering at the back!), and so that has got me into trouble a few times over the years.
There are more common examples I have trouble with, too. I often type "the" when I mean "then" and more worryingly, I often type "now" and "not" interchangeably. There's a world of difference between "the application is now working" and "the application is not working". And it's very unlikely that anyone will ever developer a contextual checker to help me with this mistake.
T9 also gets me into trouble. "He" and "If" are often found in place of each other in my text messages. And "okey dokey" is so hard to type using t9 that I usually go for the T9-suggested "oldy foldy" instead. I rather like it - "oldy foldy" sounds rather sweet, doesn't it?