Not reconciling accounts is a commmon mistake I see with small businesses.
After the end of each month your bank, credit cards and even merchants like PayPal will release statements showing your beginning and ending balances as well as all transactions that occurred in that month. Take those statements and reconcile your accounts. Make sure all of the transaction amounts and dates match up and make sure the ending balance matches up as well. Not reconciling your accounts each month can lead to errors that copy over month after month. If there was a transaction that was not input correctly and it also was not reconciled, the error may not immediately show itself and could be throwing everything off month after month.