Yes Bulletproof Coffee can cause you to gain weight instead of losing weight because Bulletproof Coffee is high in fat.
You shouldn't gain much weight from Bulletproof Coffee if you just drink the Bulletproof Coffee in moderation but if you drink too much Bulletproof Coffee the high fat content can cause weight gain.
I drank a lot of Bulletproof Coffee and did put on some extra weight but after slowing down on it I lost the weight again.