I also constantly rearrange letters in the words "marriage" and "emperor;" I have no idea why, but it just happens.
Well, "words we have trouble spelling" could be a whole other thread. "Restaurant" give me a great deal of trouble, as does "necessary" (and I've found out I'm not the only one.)
There are commonly occuring typographical errors as well;-- "you" for "your" and "hte" for "the".The study of grammatical or lexical errors is actually pretty interesting; there are identifiable reasons for why certain errors occur or why people use words (incorrectly) in certain ways. It rarely has anything to do with intelligence.
Of course, it doesn't hurt that "you" and "the" are two of the most common words in English. I've found that once something goes from being a conscious task to an unconscious task, I can get very sloppy. I had a job working at a phone bank, and I had adopted a nom de guerre (as they had advised us to do.) I found after the third or fourth call, reading from the script, I couldn't say this made-up name without extreme slurring. It was like the words were turning to mush in my mouth. Has anyone ever had an experience like this?