The "intercepted cheque" fraud.
This is an old one and is becoming less common with the advance of electronic banking.
How it works: You write out a cheque to your supplier and post it off . Someone, normally in the recipient company or organisation intercepts the cheque and alters the name ever so slightly (eg they change SARS to SAR Smith). They already have an account set up with the new name of the cheque and make the deposit. The cheque is processed. You are none the wiser, as your bank statement reflects the correct amount - how are you to know that it went into the wrong account. Your supplier will take several weeks before they start sending you overdue notices and by then "SAR Smith" has cleaned out the account and disappeared.
How to prevent it: 1) Use electronic banking. 2) If you have to write out cheques use the full name which makes it harder to alter. 3) Monitor you supplier statements. Check them as they arrive so that you can sort out missing payments straight away.