The Guardian reports that researchers at the Concise OED have worked out exactly which homophones (words that are spelt very similarly but with completely different meanings, if you weren't sure) are causing us the most trouble. Apparently the most common one is diffuse/defuse.* The Guardian has provided a list, either (it's unclear) from the Oxford people or its own database of recurring mistakes, as featured in the daily corrections column. At least Guardian writers who have seen their work featured there can take comfort in knowing that they're not alone. These are, according to the researchers, mistakes made by relatively literate people, found "in texts that are otherwise quite well spelt". They suggest that the use of spellcheckers may be at least partly to blame (surprise, surprise). Additionally, they're finding a tendency to write on the internet "as if it was a spoken rather than a written medium, with all the mistakes which arise through doing that". (Shouldn't that be 'as if it were', Mr Dictionary?)
For the few of us fortunate enough to find spelling in English instinctively easy (for me, 'diffuse' and 'defuse' sound completely different: how could I ever confuse them?), this is often less irritating than a source of entertainment, especially when the result is a good suggestive pun. But publishers looking for the successor to Eats, Shoots and Leaves may want to take note.
By the way, the same researchers have noted that the 'grocer's apostrophe' is, in fact, on the decline.
*Update: I realised that if I'd been asked which homophone I thought was the most common, I'd have plumped for 'loose' (for 'lose'), which is some way down the Guardian's list. I keep seeing it everywhere!