I don't think I used to make mistakes between words that sound similar when typing. I think it's a result of doing a lot more "conversational" typing since the advent/domination of the internet: email, IM, chat, SMS, etc. Every now and then I'll be typing something and put in a word that sounds similar; like, beyond the usual (and universally horrifying) their/there/they're. My theory is that I'm having the conversation in my head and dictating to my fingers, and sometimes they don't hear so good. Anyway, whenever it happens with a weird pair of words I think it's funny and mean to remember it, but I never do. So now I am.

(tom): i sure don't have much motivation lately
(lens): agrd
(lens): i wonder if that's what the 30s are all about
(lens): the death of useful enthusiasm
(lens): youthful