Many times, bad checks are written inadvertently by people who simply were unaware that their bank balances were too low. It is always a good idea to have a small overdraft line of credit to cover such situations, or keep a close eye on your balance near bill-paying time.
When there are insufficient funds in an account, the bank will "bounce the check" (refuse to honor it). Banks and vendors frequently charge fees for bounced checks, sometimes exceeding the amount for which the check was written. Online banking can help to avoid writing bad checks by allowing you to view your balance more frequently. Consumers can also create a backstop account that is automatically debited if the primary checking account is too low to pay a specific check.