Lost

"Lost" was one of the first serialized dramas I got really into.. (The first was actually "Profit"). One of the things that bugged me was how the show creators continually denied that they were making it up as they went along, although evidence strongly indicated otherwise especially with the unsatisfying end where they introduced a whole new storyline in an attempt to sweep all their other loose ends under the carpet. 10 years on from the show and this is even more apparent.
They had a horrible habit of introducing some interesting plot development, and then completely ignoring it for months, (or years). One of the most frustrating examples that I can recall was at the of penultimate episode of Season 2 "Live together, Die Alone Pt 1"

2 Men playing chess at an arctic station, when a computer starts to beep. They excitedly call Penny Widmore and tell her "We found it!"

This was the first time that the outside world had been shown in the present tense (not a flashback), and it confirmed that the island was not purgatory or any other number of wacky theories. Too bad this was never really adressed for an entire season 3, (except for some confusion in the Season 3 finale, over whether a boat that shows up is Penny's boat or not)