Normally stemming from either a premature cancellation, or writers writing their way into a corner.
What makes a finale a "failnale" is completely subjective, and is a decision up to each person to make. However, there are times when the writing is so blatantly bad, only the most hardcore of fans would defend it.
Note : While it mainly refers to television programs it is also applicable to, books, video games, comics, and basically any form of entertainment that eventually ends.
Steven : "That was total garbage! They didn't even foreshadow something as big as that. It just showed up out of nowhere! And did you see what they did to the villain?"
Rachel : "Are you talking about the convenient and sudden mental breakdown? It was only there so the good guys could easily win. What a cop-out"
Steven : "Yeah, it was a total failnale!"