The most common mistake I make is adding or subtracting something twice.

It can happen that the bank makes a mistake. We switched banks recently because the old bank's online banking system had some sort of glitch where every now and then it would decide that our starting balance when we opened the account was something other than what it was. We literally had to go ALL the way back through our transaction history to figure that out. I got to the very first transaction ever, and wait a minute, it was the $100 deposit to open the account and the online banking statement said our balance was $39 at that point! Since when was 0 + 100 = 39??

Another thing that can help is go do something else, take a walk or go for a ride, and come back to it in a bit. How many times have I torn out my hair trying to find the mistake, I finally have to go fix supper or something, and when I get back to it the mistake just jumps out at me. Where the heck was it hiding the whole time I was searching for it?

Good luck, hope you get it figured out.