This happens to me all the freaking time, and I bet if I searched my own posts here I could find at least a dozen examples. But it's not only when I'm typing the post out - it's also immediately after typing it. I'll make a post here, then go on fiddling with the problem, and voila! I figure it out.
I find it very annoying, because sometimes I've been at the problem for a week, and sometimes just a few hours, but regardless, until I post a question here, I'll NEVER EVER solve it.
EDIT: Figured it out!