Development Hell: The original game was almost killed twice. The first time was the Gollop brothers fault when they submitted a confusing, short design document to Microprose. The second time, Spectrum Holobyte actually cancelled it, but Microprose UK more or less ignored it; when the publisher was looking for a March 1994 release title, Microprose UK stated, "Hey, look what we've got!"
Showing that Tropes Are Not Bad, the original X-COM actually owes many of the things that make it famous to Executive Meddling. Initially, the game was going to be a pure tactical combat game set in an alien world; executives demanded the addition of the larger strategical game to tie things together and asked that it be set on the more familiar setting of Earth instead.
Then showing that Tropes Are Not Good, Interceptor and Enforcer were mostly the brainchildren of MicroProse executives, rather than the core X-COM team, who wanted to make Alliance and Genesis, but were forced to make the (universally-agreed) sub-par games to placate their overlords.
Springtime for Hitler: It's more or less an Open Secret that Interceptor and definitely Enforcer were this on Hasbro's part, having bought up Micro Prose simply to eliminate competition and intentionally running the acquired IP into the ground rather than shift focus away from their own core works.
X-COM Alliance, a squad-based FPS where a team of X-COM soldiers got teleported away to another world, where they have to work together with the friendly aliens to survive against hostile aliens. An unfinished build of the game was discovered in late 2011.
X-COM Genesis, returning to the gameplay of UFO Defense.