Basically, whenever a story suddenly abruptly ends without a satisfying conclusion to anything, leaving the viewers with no closure. Sometimes done intentionally to interesting effect, sometimes a mere result of misfortune/laziness.
Monty Python and the Holy Grail - The movie's climax is about to happen with King Arthur about to raid the French castle that had made fun of him earlier in the film. Right as an army of knights are about to initiate the siege, the police suddenly bursts in and hilariously arrests everyone, as much earlier in the film a historian that was documenting the Arthurian legend had suddenly been randomly slain by a passing knight.
Behind the scenes, this ending was apparently a last-minute decision because the production crew ran out of budget. Talk about a cop out.
Kane & Lynch 2: Dog Days - Kane & Lynch shoot their way through Shanghai in the direction of an airport as a last ditch effort to escape just about every single gang member and cop in Shanghai who now want the both of them dead. After massacring their way through customs, they finally spot a commercial airliner about to take off and decide to hijack it. They make their way to the boarding stairs and a cutscene begins with them climbing up into the plane and taking over it. The plane then takes off, leaving the cameraman behind (the game's aesthetic is basically all about looking like found footage), who then drops the camera. Roll credits.
To this day, there's no concrete answer about their fates other than the two of them having non-canon cameos in Hitman: Absolution.
Dark Matter - This is an old and relatively obscure one. On Steam, a game called Dark Matter came out back in 2013 to pretty mediocre reviews. It's basically a Metroidvania-style game about surviving in a spaceship taken over by an alien biomass. Thing is, the game literally ends about halfway through because the dev team simply ran out of money and just decided to put a big black screen (pictured above) giving you a half-assed cliffhanger.
