Jamesaritchie:
My biggie is ending sentences with prepositions. I know the rule, but I seem to look right past them unless I sit the manuscript aside for several days or more.
There is nothing wrong with ending a sentence with a preposition. If it sounds right, it's fine. It is a question of style, rather than grammar. Grammar (or style) should not be followed religously; it should be made use of where appropriate.
What did you go out for?
For what did you go out?
Honestly now, which one sounds better? (I'm sure there are some people would still feel the latter sounds better. But the vast majority will feel the former flows more easily. [oh look, I started a sentence with a conjunction - oops])
What always amazed me was how many manuscripts came in with you're/your. two/to/too, and their/there/they're mistakes. It doesn't take many of these early in a manuscript to cause a quick rejection.
This is often a form of dyslexia. (Or maybe it's dyspraxia; I'm never quite sure which is which.) While most people know full well the difference between these homonyms, when typing (or writing) quickly it is very easy to muddle them up. In addition, it is also something that can slip through when you reread, because you know what you expect it to say from the context. I suffer from this problem (as well as getting p and d and b and g/q mixed up when writing by hand). My emails (and posts on boards such as this one) are full of these mistakes (and no/know our/hour and plenty of others). However, I do usually manage to spot them on a reread (especially if I haven't look at the writing for a day or two).
The other problem, of course, is that spellcheckers will not pick up on these errors, because they are all correctly spelt words - just used in the wrong place!
When I'm editing I tend to forgive these kinds of mistakes, but get annoyed by others instead. I hate apostrophe errors most of all, but will usually forgive an its/it's error. It's odd. I suppose we all have our pet hates!