Hello Rich!
Please find below concise answers to questions you asked.
After reviewing each section of this recent CAT, how many questions did you get wrong....
1) Because of a silly/little mistake?
Of the first 10 mistakes, 3 were wrong (i.e. 3rd, 5th, 7th) - all of them were silly mistakes. When I read, I sometimes fail to read the question correctly, i.e. I fail to notice crucial words, such as 'odd/even'.
2) Because there was some math that you just could not remember how to do?
This may not be the case. I mean, I knew how to do these Qs, but I failed to remember some basics. I mean I generally remember these rules but I didn't realize, while handling the question, that I was supposed to make use of a particular knowledge/rule. ex. that the remainder, when a number is divided by 7, must be in 0 to 7-1 range. This is obvious but I somehow didn't remember this type of basics when undertaking a question.
I've always been good at math: combinatorics and probability, which is pure math, have always been easy to me. But I often fail to remember crucial rules.
3) Because the question was too hard?
I encountered one question that I couldn't handle at all. This was the first time I saw this type of question.
4) Because you were low on time and had to guess?
I had ~15 minutes for last 3 questions, so I focused as much as I could and exploited the remaining time properly.