Count It Once?

In every retail job I've had, I've counted the money in the tills. This includes my very first job ever: video store clerk. I was in high school at the time, and I remember telling my mom that if it was right the first time, I only counted it once. If it was wrong, I counted it a second time. She was surprised that I sometimes only counted the money once. At the time I didn't have an explanation -- it just seemed logical to me -- but now I've got one!


So, Mom, here it is:

What are the odds that the amount of money in the till is wrong, but that I count it wrong and get the right amount? For example, let's say the register was supposed to have $527.89 in it, but someone messed up earlier in the day and it actually has $527.83. It's not very likely that I would count the money wrong and happen to get $527.89. See?


My rule of thumb is this: I want the right answer once or the wrong answer twice.