- "Frozen donkey wheel" redirects here. For other uses of "Frozen donkey wheel", see Frozen wheel.
Code names have been used by the producers and writers of Lost to refer to season finale cliffhanger scenes, often containing plot twists. The code-named scenes are left out of the scripts for all but essential cast and crew.
The code names follow two patterns: the first two were both types of breads and had no relation to the final scenes; the second two were odd phrases that initially seem like non sequiturs until seen within context of the final scene.
So there's kind of been a little bit of variety. We started with some Jewish bread products and then we moved on to sort of more esoteric terminology.
Season 1: "The Bagel"
This code name appears to have no direct relevance to the final scene; however, the bagel, a type of Polish bread, is a traditional part of American Jewish cuisine, just like "The Challah".
A possible explanation is the word "bajel", which in Spanish is a kind of ship (referring to the Fishing boat).
This is one of the only two code-named finale scene not to refer to the last scene of the finale, but was actually seen several minutes before the final scene.
Season 2: "The Challah"
The code name for Season 2's final scene was "The Challah", which revealed Henrik and Mathias in the listening station, at a desolate, snow-blanketed location. Upon receiving a computer notification of the detection of some sort of electromagnetic anomaly, they phoned Penelope Widmore to tell her that they had "found it". ("Live Together, Die Alone") (Official Lost Podcast transcript/May 19, 2006)
This code name also appears to have no direct relevance to the final scene; however, the challah is a type of bread which is traditional part of Jewish cuisine, just like "The Bagel".
Season 3: "The Rattlesnake in the Mailbox"
Season 3's ending code name was "The Rattlesnake in the Mailbox",  also known as "The Snake in the Mailbox", where Jack, drunk and depressed, meets Kate at an airport, pleading with her that they have to go back to the Island. The scene revealed that the episode's flashes have actually been flash-forwards, as opposed to flashbacks. ("Through the Looking Glass")
While not a literal reference to the final scene, the code name shares two similarities with the feeling and mood of the episode's final scene. First, the scene was surprising like a snake in a mailbox; and second, it was a plot twist, being that the audience had up to that point been accustomed to seeing stories told only in real-time or in flashback.
The code name may be a reference to the cult Synanon, which attempted to assassinate an enemy by putting a live rattlesnake in his mailbox. The cult was based in Santa Monica, CA, where Bad Robot Productions is located.
Season 4: "The Frozen Donkey Wheel"
Season 4's finale scene code name, "The Frozen Donkey Wheel", was actually the reveal of which character was in the coffin first seen in "Through the Looking Glass, Part 1". Damon Lindelof reveals on the DVD commentary for "There's No Place Like Home, Part 2" that name was chosen as a decoy to spoiler posting fan sites, as an earlier scene showed a literal "frozen donkey wheel" and would throw these sites off should that scene be leaked.
Season 5: "The Fork in the Outlet"
The code name for the secret scene of Season 5 was "The Fork in the Outlet". In the official podcasts leading up to the finale, producers Damon Lindelof and Carlton Cuse referred to the code-named scene as the "secret final scene", which would indicate Juliet detonating the plutonium core of the Jughead bomb. However, at the The Fuselage, the official site of the creative team behind Lost, actor Jorge Garcia stated that the scene which was cut from the script was Ben stabbing Jacob, and "Locke" kicking him into the fire.  ("The Incident, Part 1")
Season 5 naming process
On the March 19, 2009 Official Lost Podcast, Damon Lindelof and Carlton Cuse invited fans of the show to submit their own code names for the final scene of the Season 5 finale. The two producers nominated their thirteen favorites  from a shortened list of best submissions narrowed down by Official Lost Podcast producer Kris White, and podcast listeners voted for their favorites by e-mail.
"Spin Drift Beck", from Dallas, Texas, submitted the winning title, "The Fork in the Outlet". Matt Mitovich revealed in the TV Guide podcast that Lindelof and Cuse also chose "Invisibul Dinasaur Hed" [sic] as the number one rejected entry. A campaign had been run by participants of Lostpedia's forums to make this meme the winning name. 
Season 6: "Sun and Jin's Wedding"
Damon Lindelof and Carlton Cuse stated the Season 6 finale, also the series finale, would not have an official code-named scene because they didn't believe there was a single scene that lended itself to being named.  Actor Jorge Garcia stated in his March 31, 2010 Geronimo Jack's Beard podcast that the final act of the episode was left out of his script.
The scene that was withheld from every script (except those of the participating actors in the scene and a select group of crew members) was not the final scene, but rather the penultimate. Only actors Matthew Fox and John Terry were privy to the information disclosed within the scene. It consisted of Christian Shephard helping his son Jack realize that they were dead. This revealed to the audience that the flash sideways, instead of being a new timeline created by a nuclear bomb (as the audience was led to believe), were the characters' afterlife.
The actual final scene, when the characters reunite in the church to move on, was originally described as "Sun and Jin's Wedding" in call sheets and other related crew documents to hide the truth of the scene. (Geronimo Jack's Beard)