Oh man… next time I move, I want to write “Fraggle” on all of my boxes!
I’m surprised no-one has mentioned ‘teh’ yet. 
Of all the words, it’s ‘occasion’ and its derived forms that trip me up all the time. There is something in me that wants, with the desperate intensity of a moth being drawn across the dark tundra to a Russian nuclear-powered polar lighthouse, to spell the word with an extra ‘i’ inserted after the first ‘a’.
Discrete for discreet makes me all stabby.