If you have written a check that hasn't cleared your checking account, the cash is still available for other transactions, such as ATM withdrawals or debit card purchases. So if you're not careful and don't leave enough money in your account to cover the check, and you don't have an overdraft account, your check will bounce. And if a check bounces, you'll be charged an nonsufficient funds (NSF) fee.
NSF fees are especially tricky because the order in which the bank processes each check determines the amount you're charged. Let's assume you have $80 in your checking account and you write three checks for $20 each, followed by one check for $70. If the bank processes the three $20 checks first, and the $70 check last, you will bounce only one check and therefore be charged just one NSF fee. However, if the bank processes the $70 check first and then each $20 check, all three $20 checks will bounce, leaving you with three NSF fees, one for each of the $20 checks.
If you're hit unexpectedly by nonsufficient funds fees, call your bank and request a refund. If you have a good account history, the bank may forgive some of the charges.