I have the same symptom. My mother always thought it was odd that when asked to spell things out loud that I didn't spell in syllables, but I'm basically reading what I've written down on the virtual piece of paper in my head, so syllables are irrelevant.
Same here, too. When I find I've got a word wrong, assuming it's not just due to the font size in the BBS edit box being too small or to an editing error like "well" above, I usually find that I've always got that word wrong, that I've been faithfully remembering an incorrect spelling.
I've always figured it was some sort of idiot savant type of thing.
What's interesting is that this ability -- to look at an entire word and see whether it's correct or not -- is often described as exactly the ability that dyslexics lack. It sounds as if dyslexia is a less-than-average facility at an innate skill which it's also possible to have a greater-than-average facility at.
Peter (resisting the temptation to reword that sentence with "at which...")