When spelling checker fails to pick up an incorrect word because the word, though incorrect in your sentence, was actually a real word in itself.
These require serious proof reading to find, as they won't be underlined in red, however, it's worth doing because they can make a HUGE difference to what it is you were trying to say...
The pseudo-typo is a real word that is inappropriately placed:
"We all lined up on the star line."
(We all lined up on the starT line)
"If you do that again, I'm going to come down there and kiss your ass!"
(If you do that again, I'm going to come down there and kiCK your ass!)
"I wrote to my boss and made an horrific pseudo-typo, instead of saying I'd like to kill him, I said I'd like to kiss him and now we have a date on Wednesday :( "