a daily basis (if not more frequently), especially if you
have a credit or debit card tied to any of your accounts.
Trust me, I speak from experience. While online yesterday, looking at my balance and transaction information, I noticed that one of my accounts had outstanding card charges in excess of $3500 -- far, far more than I ever use on that card. As soon as the detail screen opened up, it was immediately apparent that my card had been hacked. It started off with rather small charges to a foreign Paypal account -- obviously not mine, and continued on with increasing charges for the same description.
Fortunately, those charges were all made yesterday, so checking out my online accounts really paid off, as the bank was able to cancel that card and send me a new one before any/many more charges appeared.
If this ever happens to you, call your bank's local branch, or the telephone number on the back of your credit/debit card (you may need a magnifying glass...), and ask for the Fraud Department (or press the appropriate keys on your phone). Tell them your card has been hacked and ask them to cancel the card and send you a replacement. If you have access to your charges, tell them which ones are legitimate and which ones are fraudulent. They should put a hold on those charges that you claim are fraudulent and put that money back into your account. You may need to follow up the next day to make sure the charges have been reversed, as well as to see that they have allowed no further charges to your card. Also, change your passwords and/or PINs related to that account.
I hope this NEVER happens to you!