A FAVORFUCK is when a friend does (or attempts to do) another friend a favor, but in actuality they end up fucking that person over royally.
When it's called out, the favorfuck can lead to especially viscous confrontations, because both parties involved will vehemently stand their respective positions. The favorfucker truly believes that they did the other person a favor, and they are shocked by the audacity of the other for that person's "ungratefulness" in confronting them after doing this favor. On the other hand, the favorfuckee is angry because they were blind-sided by a supposed favor (to help them out in time of need) that turned out to either not help at all or (more commonly) make their situation substantially worse.
A flagrant (aka first-degree aka or felony) favorfuck is when the favorfucker either did not have best intentions OR (more commonly), despite their good intentions, they exhibited gross incompetence in executing the favor, and as a result ended up completely fucking over the other. In this case the favorfuck was clearly avoidable by the favorfucker. In contrast, an innocent favorfuck is unavoidable, purely accidental, and circumstantial: when the favorfuckee truly had the best intentions AND they were competent in executing the favor, and it was just misfortune that the results of the favor ended up being negative in the end.
Your car broke down and you need a ride, as you have to be somewhere important by a certain time. You call a friend to ask the favor of them coming to get you, clearly explaining that you can call someone else if they can't help this time. They insist they can, so you don't call anyone else. Then, that person takes an excessively long time to come get you, and you end up being late to your engagement and then your totally fucked. The situation coulda been easily avoided, had the favorfucker declined the favor because they knew they could not execute adequate favor-follow-thru. The favorfuckee could have then called one of many other friends who likely could have executed the favor better. The favorfucker is shocked by the other person's "ungratefulness" while the favorfuckee is angered by the other person's inability to execute the favor properly. So everybody loses in the end: the favorfucker got zero favor-points for doing the favor, and the favorfuckee got fucked over expecting a helping hand.
Larry David's fictional character (creator/star of HBO's Curb Your Enthusiasm) is all about the favorfuck.