A team of editors and lexicographers at Collins dictionary publishing house claims that supersede is the most misspelt English word. It is spelt wrongly 10% of the time, they say, which I think is pretty good going. I used to mark GCSE papers and reckon that the word restaurant was misspelt by well over 50% of candidates.
The reason we find it hard to spell supersede correctly, writing a c instead of the second s, apparently, is that we are influenced by words such as precede and intercede. It's quite true that supersede is a one-off. There are just eight words in the Oxford English Dictionary ending in -sede and all but supersede are obsolete. But what great words they are! There's essede, a war-chariot used by the Gauls, obsede, an old version of obsess, possede and repossede, old forms of possess and repossess, sede, an old version of seat and ypurchasede and ypressede, Middle English forms of purchased and pressed.
In Middle English, which was spoken between the 12th and 15th centuries and was the language of Chaucer, past participles began with the letter y (eg ybegunne, yfallen). The y spelling developed from the Old English prefix ge- (eg geboren, which became yborn, meaning born).
Another of the top five misspelt words, according to Collins, is phlegm, but how many instances are they talking about, I wonder. How often would your average person want to write about phlegm?
In my research on crosswords, it's interesting to see which words fox many people. They often give up on a puzzle soon after getting the answer broccoli, professor, harass, tariff or another word where there is doubt about whether there is just one consonant or whether it is doubled. The commonest mistake is to get the single and double consonants the wrong way round. The resulting word fits the space for the answer okay, but makes adjacent answers impossible to get!
As for the top five spelling mistakes, my guess would be that the answer would be different for each profession. The top howler made by estate agents has to be 'sort-after' instead of 'sought-after'.