You really should balance your checking monthly. This happens more frequently than you think. It is typically caused by human error at the point of sale.
I typically have to dispute a few transactions every year since I use debit quite a bit.
Most common offenders are Gas Stations and Fast Food. Take note anytime they have a glitch or have to swipe your card more than once. This is when the double charges occur most frequently.
Another issue I have come across is places who do not remove holds. I had a 100 dollar hold on my account for about six months once. I finally called the bank and it turned out some hotel never released the 100 deposit hold on my account. It most likely would have stayed there indefinately had I not called.