Metal Slug Advance

Metal Slug Advance, the greatest source of disappointment known to man.

Unused Sprites
There's a lot of unused animations for the player characters, mostly the likes of reloading and idle animations that would have been used for flavour.

Tyra firing while crouching. This is impossible to do in the game.

As for Walter, there is also an unused firing while crouching animation.

An unused animation of Tyra putting makeup on.

Unused Objects
A fair few remnants of the scrapped underwater stage remain, like the Slug Mariner, squids, floating mines and perhaps more.

Unused Area
Every single preview document about the game in its early days makes mention of how there are six levels, are has numerous references to "underwater waterways". No such area is visited, and although there are technically six stages, the last one is effectively a bonus area that's not counted on the save file stats.

Debug Mode?
There's a few textual remains of a debug mode, most of it nigh-identical to that of the arcade games, as well as the numerical listings of a stage select. Mission 3 has ten areas (only seven are seen in gameplay, including a cutscene) and Mission 4 has seven, meaning two areas aren't accessed.