It's rude to promise something and fail to deliver.
Cam gets a call from his friend Adam who asks to come pick up the lawnmower Cam promised he could borrow.
Cam wants to tell Adam not to come over.
Cam tells Adam not to come over, that he'll bring the mower over to him right away.
Cam tells Adam not to come over because he changed his mind and hangs up the phone.
Adam is delighted because not only will he get the mower but Cam will deliver it, too.
Adam is livid that Cam could so flippantly just renege on his promise like that and be so abrupt.