Asklemmy

I once worked for a small ISP that decided to enter the calling card business. I built them a voice prompt system on top of Asterisk that made received PSTN calls over PRI and made outbound VoIP calls, all metered to cards with a unique number and a balance, and a UI to activate them. The business got boxes of physical cards printed, with a plan to sell them to convenience stores.

They hired a salesperson (AKA worst coworker) to sell the boxes of cards. This coworker then sold many boxes of activated cards to many small stores at an unauthorised discount (below the level of profitability), for cash rather than the approved methods for retailers to buy them, and then apparently spent said cash at the casino. The business had to honour the cards (i.e. not deactivate them) at a big loss to avoid ruining their reputation, since the buyers apparently did not know the deal was dodgy. His tenure was, suffice to say, not long, but in his short time there, he managed to put the business under financial strain and it eventually went into liquidation.