I find that I have moments when I, inexplicably, go blank and completely forget how to spell a simple word. So, if I see one, or two, misspellings in a post (for example), I assume that that is probably what has happened to the poster.
I also, sometimes, spell something correctly, but it looks wrong to me! I then proceed to try to spell it other ways, which generally look even worse!
Also, there are obviously the spelling differences between English and American English and sometimes I find myself using an American spelling (which can look a bit odd, in amongst the other English spellings).
I don't like it when I buy a book and there are misspellings, though! I've paid good money for that book and I should be able to expect that it has been written by a fairly literate person; or at the very least, properly proofread before being published!