As a writer I'm interested in words, and I expect that most publishers have a similar affection, so I was slightly surprised when I got an invoice from Macmillan (I'd bought some copies of one of my books - someone has to) and found what appeared to me to be a spelling mistake. Here's what I saw:
The invoice used the spelling 'despatch' several times, yet I've always spelled the word 'dispatch'. As often happens at this point I had that nagging doubt I'd been doing it wrong all this time... so I hared off to that universal arbiter of all things wordly (sic), the Oxford English Dictionary.
Even though the dictionary gave both spellings, I was delighted to discover that my spelling is the more correct one - and the incorrect (sorry, less preferred) spelling seems to have been due to a late night on the part of Dr Johnson.
According to the OED, the word was always spelled 'dispatch' from its introduction until the early 19th century. However, Johnson use 'despatch' in his dictionary. This sounds almost certainly a mistake (we've all seen what he was like on Blackadder) - apparently Johnson only ever used 'dispatch' himself, as did all the authors cited by him, so it's hard to imagine it was intentional.
I leave the OED to give the final word: 'dispatch is to be preferred, as at once historical, and in accordance with English analogy; for even if this word had begun in Middle English with a form in des- from Old French (which it did not), it would regularly have been spelt dis- by 1500: see des- prefix, dis- prefix, prefixes.' Can't argue with that. So if you feel the urge to 'despatch', correct yourself immediately.