I made a really dumb mistake in another post: I wrote something was given a free reign when I should have said free rein. It wasn’t because I don’t know the difference between the two (I do), or because I had been working all night (I had). I wrote it because I was thinking and typing at normal speed, and that’s just what came out unconsciously. :smack:
Is there a name for this phenomenon? How often does it happen to other people? 
I usually call it “oops” or “fucking up a little.”
It’s called “misspelling” and it’s very common when intelligent people with large vocabularies aren’t paying sufficient attention and substitute a homophone. Relax, it’s normal.
I only get concerned when I make these kinds of errors in a work email.
I forgive you, terentii. What’s important is that you understand what you did wrong, and you’ll try not to do it in the future. 
Moving from Cafe Society to MPSIMS.
I call 'em “word-os” instead of typos; I often make wordographical errors. 
Eye donut no watt you’re righting a bout.
What I hate is when I accidentally a word without realizing it. Even upon rereading I may not notice, because I know that word is supposed to be there.
This happens to me a lot. Apparently my brain is thinking faster than I can type, so I type the next word instead.
BigT is one of those subtle comedians.
If it makes you feel any better, I just totally forgot how to spell ‘‘impermanence.’’ And I almost always spell ‘‘disappear’’ wrong. (Well, I rarely spell it wrong anymore because I have written it so many times while explaining that I can’t spell it.) Those aren’t even the dumbest things I’ve done today. It’s okay to be unperfect.
Iv’e nevver maid eny speling oar grammer misteaks sew eye wooden no.
Just bless you for knowing there is a difference!
OP, what you are describing is a serious medical condition. Please go talk to your doctor, lawyer, and astrologer about this right away.
Apparently it’s very common.
I c
[In the voice of Leonard Nimoy’s Mr Spock]: Thank you, drewtwo99. I shall seriously consider your advice. 
In Lincoln at Gettysburg, Garry Wills mentions a similar phenomenon that he calls “dittography” in describing how Lincoln would write out copies of his Address once he was back in Washington: apparently, Abe would unconsciously repeat or omit words, or inadvertently use synonyms that were almost interchangeable with the text’s original words, but not quite. Wills explained this by saying that the mind races ahead of the eye while writing, and we don’t notice the differences until after we’ve already written them.
In other words, we all do fuck up on occasion. 