The best thing about online banking is that you can see mistakes as soon as they happen. You don't have to wait for an incorrect bill to show up in your mail, then take it to the bank, wait for them to process it, wait for them to mail you back indicating the correction has been made.
For example, this morning I noticed that I was charged interest on a loan that has already been paid off. I called my branch, informed them of the error, and the mistake was fixed over the phone.
It took a total of five minutes.
Everyone should do business this way. It was quick and painless for both parties.