And then there are the words that I think are different but turn out to be acceptable variants of each other.
For example, I always thought loath was the correct spelling of the adjective ("She was loath to admit it"), and that loathe could only be used as a verb. Not so; MW10 lists loathe as a variant on loath. (Though loath can't be used as a verb, so the two aren't entirely interchangeable.)
See my entry on forgo for another example of me being pickier than the dictionary considers necessary.
Another example: I always used to be in the camp that believed that enormity should be reserved for monstrous evil, rather than referring simply to great size (enormousness or, better, immensity). Sadly, MW10 doesn't back me up on this; my prescriptivist tendencies are once again hung out to dry.
And really, that's probably the eventual fate of all of these words-easily-confused. Dictionaries are intended to be descriptivist; they serve a prescriptivist role, but y'know, if everyone starts to spell lightning as lightening, the dictionary should start to list that as an alternative.
But I do still note, when editing, that a certain class of intellectual snobs like me will wince at what they believe to be misuses of words, even if they/we are just behind the times. So if you're loath to see prescriptivists sneering at you, forgo the traditionally frowned-upon uses of enormity and other such words.
(Entry edited shortly after posting to correct misspelling of prescriptivists. Life is all ironical 'n' stuff.)