In Animal Crossing Gamecube, the "Dummy" Item escaped though a mistake in the igloo trading game. It has the appearance of a white triangle with the English word "dummy" written on it in katakana.
Similarly, if one hacks an item not meant to be in the game at all, and then drops it outside, there is a chance it will either A) become invisible and thus impossible to pick up again (and also make that tile unusable to drop any other item on it since it's "used"), B) shuffle among the icons of other droppable items (such as a bag, a pitfall, furniture leaves, etc), or interestingly, C) become a fish frozen in-mid flop. That last is a remnant from the original Japan-only N64 version, in which fish would flop over land into the nearest body of water when dropped instead of flying straight in.
There is also a list of 60 or so other left out items, they have names, but are otherwise exactly like the DUMMY item. Some of them would appear in future games (massage chair, spa bath, lucky cat), while others are just plain bizarre! (moon dumpling, unused monkey, robotic flagman)
A game-breaking item exists in the Gamecube version as well, dubbed the 'paper airplane'. Dropping it outside places a little paper airplane on the ground, whats so bad about that? Well, exiting and entering a building creates a duplicate, entering and exiting again creates two duplicates, followed by 4, then 8, and now I'm sure your starting to get the picture here. They will eventually spread like wildfire, locking up the whole acre, and if it continues, your whole town! And don't bother trying to pick them up, they'll appear to be gone, but they are still there and will come back, complete with multiplying, when you enter and exit again.
Supposedly the basegame of The Sims 2 was supposed to have weather, but the code was commented out when they couldn't prevent the rain from falling through the roofs of buildings. An expanded version was wrapped up into the Seasons expansion pack. Pre-Seasons houses are not all weather-proof, though, and many have to be altered.
An Expansion Pack introduced the object magic lamp, which when rubbed summoned a genie. The player could ask for something predetermined, e.g. love or beauty. There was code that would supposedly give the option for sims to change their age bracket, but that was dummied out.
There also is a Debug mode that is only accessible by using a cheat code in Create A Sim which allows you to access hidden outfits that for whatever reason are not available in the game (along with NPC outfits and career outfits).
SimCity has its fair share of hidden content, some of which was discovered by mere accident by modders:
In 3000, some of the game's adviser panels would display buildings that didn't exist or couldn't be found in the game; this included what appeared to be a smaller version of the Maximum Security Prison, a variation of the apartment building Rock Bottom Terrace, and some bizarre rocket-ship-shaped building.
The SimCity 2000 theme is still in 3000, but can't be found in the music list. Additionally, the track "Concrete Jungle" that was in 3000 was for some reason removed from the SimCity 3000 Unlimited playlist. Some judicious editing of game files will fix both of those.
Some hidden contents in 3000 can be accessed through a cheat. This includes the SimCity Castle and tiles to make real-size airports (compared to the surrounding buildings) instead of the small-scale airports. It's also possible to get a very useful 'Better Bulldozing'-tool through the editing of a configuration file, which allows things like half-highways. Furthermore, by importing a grayscale file you can have maps that are impossible to create in the map-editor.
In Sim City Deluxe (as well as the original game and separate expansion pack), there was supposed to be some extra tools that would allow you to change the color of the terrain, create lakes at various altitudes (like in a mountain gorge), and plant various species of trees, but weren't available in the final product (mods do offer some of these, but most aren't permanent and won't save in the city). And if you also look at Rush Hour's additional music tracks, you'll find that some numbers aren't there (it appears that there were going to be about 15 new music tracks, but not all made it). There also appears that there was some kind of on-screen advisor or some kind assistant who would point things out that had a full body model and even UI panels, but he shows up no where in the actual game.
The Sim City 4 expansion came with an unfinished transportation network. It's impossible to mod a new "real" transportation network, but this unfinished network allowed a major mod.
It also contained a particularly dismal music file that can't be accessed in-game through any means. The haunting nature of it greatly contrasts the game's otherwise upbeat soundtrack.
Wing Commander Prophecy was originally slated to have multiplayer capability, but development resources prevented it from being finished. Unfortunately, the work had proceeded for long enough to have made it into an ad for the then-upcoming game. A fan Game Mod later added the capability to play against others online.
X3: Terran Conflict has a couple dozen starships that were not included in the final cut of the game, though only three of them were actually finished (the rest were still placeholders). The ATF Valhalla and Woden made it into the next game, while the Xenon Unknown Object was a leftover from the previous one. The Pirate Caravel was removed in a patch because it had a rather serious bug that Egosoft apparently had trouble fixing.