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1.
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:

Innocuous:
"We all lined up on the star line."
(We all lined up on the starT line)

Disastrous:
"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!)

Example:
"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 :( "
by Beetlemama March 01, 2010