Check your statements
If a bogus transaction gets posted to your bank account or credit card, you have almost complete protection--just notify the bank that the transaction wasn't authorized and they have to give you the money back plus interest. The catch is that you have to notify the bank promptly--typically within 30 or 60 days--after they send the first statement that shows the error.
If you think about it, this is incredibly powerful. If it weren't true, any money that you deposited in the bank would be vulnerable to being snatched away by fraud or by error.
Unfortunately, a lot of people don't bother verifying their statements. This is kind of understandable--I've received thousands of bank and credit card statements over the years, and I can count the number of errors on one hand. Scrupulously checking each one can seem like quite a bit of effort for not much return. Still, an uncaught error can cost you hundreds or thousands of dollars pretty quickly. Worse, it's common in certain kinds of fraud cases to make a very small charge and wait to see if it's detected. If not, the fraudsters can start adding on charges with some confidence that they won't be caught.
I attribute part of my good luck in avoiding fraud and errors to careful checking of statements. It's worth the trouble.