A day editing, and learning how to do an á on a Mac...
Something you notice editing (which you might not just from writing) is other people's blind spots. Lots of people join up some words, like "anymore" and "allover", while the Doctor is good at separating words like "how ever" and "further more". It's refreshing to know other people make the same sorts of basic error as I do.
Those who've proofed my stuff will know I mix up the homophones you're and your, and their, they're and there. I think this is because I hear the words rather than see them. I've also a gift for transposing numbers.
The thing is to be aware of your own blind spots: if you're checking for them yourself, they're no longer a weakness. Though that implies you re-read what you're writing before sending it in.
Which, editing other people's stuff, you realise isn't what all authors do.