7. Lost Producers Don't Understand Puberty
You knew this one was going to be here, didn't you? In the first season of Lost, one of the big mysteries was centered around Walt, the young boy who was stranded with a father he barely knew when his mother died. Through flashbacks, it started to become clear that Walt had some kind of psychic powers that caused exotic birds to randomly appear and commit suicide (really).
With that in mind, it made sense that when Walt and his father Michael were among the castaways who tried to find civilization on a raft, the evil "Others" kidnapped Walt to do weird island magic experiments on him. It got weird when Walt wasn't rescued until the end of the next season. His scenes were carefully shot to avoid lingering on him too long, and he promptly left the island. The end.
Malcom David Kelley, who played Walt, was about 12 years-old when the series started. Lost was somewhat unique among TV shows in that as far as its internal timeline went, a few days at most passed between most episodes. It wasn't long before Kelley started going through puberty and had a huge growth spurt that defied the internal logic of how much time was passing.
Producers Damon Lindelof and Carlton Cuse panicked and wrote him off the show. They've never explained why, on a show about a magical island, they couldn't find a way for him to stay, much less how they were unable to anticipate such an obvious development.