usually due to hitting the wrong key on my calculator. The errors are never huge though, usually just a few cents as a large dollar amount would jump right out at me. Any mistakes will come up when I balance my checking account with the next statement. I also record all entries in pencil to make any corrections easy. Of course, as I said earlier, I still use my check register. I don't care who here makes fun of me for doing it.
Right now, your checking account is probably a mess. My advice would be to go to your bank and have them balance it for you. They will charge you a small fee, but it will be balanced. Then, start keeping a running record of all transactions from now on. Record all checks, ATM withdrawals, automatic withdrawals if you have any of those coming out, deposits, etc. Record everything! Then, when you get your statement every month, sit down and balance your checkbook. Whether you use a spreadsheet or even your checkbook's register doesn't matter, it's a simple process, but you need to do it.