My son is taking the cogat this week and today they did three sections. He came home telling me that on one section he messed up filling in his answer blanks and didn't discover it until he was finished and had no time to correct it (he had a few extra questions and no blanks left). I'm guessing most of his answers were off. He didn't say anything to the teacher.

Grrr... It really doesn't matter and there's no going back. I was really interested in how he scored though, and I hate for that to be in his school record. His point to me was that it was his own error that caused the problem- he felt like he did really well until he recognized the problem. Anyone been there/done that?

Yes, sort of, ds9 took the cogat last year as part of the group of kids who already tested pretty high on other things, to see about getting into the county gifted program.
He said there were a couple sections he liked, but one he got 'really confused' on. I am assuming it was the quantitative section because that is the one which came in around 35% (!)- iirc.
There isn't really any redo other than some additional testing, but maybe this won't happen at all to your ds. (Additional testing can be done on your own, or if the school will take the request, they might have other testing they can do).
The other thing my ds does is get stressed about stuff he thinks might maybe have happened...but often turns out not to have happened! So, I didn't know whether to worry. Having extra questions at the end doesn't sound promising...

Anyway, we had some additional testing done for a variety of reasons and everything worked out well.

I'm a little surprised they had him do 3 sections in 1 day. I think there are only 3 sections (?) Maybe it was 3 sub tests. Ds had verbal on one day, quantitative on another, etc.