Super Smash Bros. Melee/Non-playable stages

These stages are only found in certain circumstances, and cannot be played in Vs. mode unless Debug mode is used (Note: Goomba, Entei, and Majora's Mask cannot be played even with debug mode, though they can be via other hacks).

None of these names are official.

Mushroom Kingdom Adventure (This is played as the first level of Adventure Mode.)
Underground Maze (This is played as the third level of Adventure Mode.)
Escape Shaft (This is played as the eighth level of Adventure Mode.)
Race to the Finish (This is played as the ninth level of Classic Mode.)
Target Test (This is played as the third level of Classic Mode and varies from character to character. Note: Debug Mode allows people to play in a Sheik Target Test Arena, which is unfinished. It is rumored that originally when someone turns into Sheik in Zelda's target test, this would appear for an easy 3 targets)
Goomba (This is played in Event Match #14 "Trophy Tussle".)
Entei (This is played in Event Match #26 "Trophy Tussle 2".)
Majora's Mask (This is played in Event Match #47 "Trophy Tussle 3".)
Trophy Collector (This is played as the sixth level of Classic Mode)
All-Star Teleporter (This is played between every stage of All-Star)
F-Zero Raceway (This is played as either the fifteenth or fourteenth level of Adventure Mode, depending on whether or not the player fought Giant Kirby. This is also played in Event Match #33 "Lethal Marathon".)
Homerun Stadium (This is played in the Homerun Contest.)
Test Stage (Apparently a stage designed for testing the Super Smash Brothers: Melee beta.)

There are also four other stages, only available through debug mode, though in such an unfinished state that the game freezes when the game starts, and thus the stages can never be seen. Their names are:

AKANEIA (One of the first Fire Emblem games was named Fire Emblem: Akaneia, so this is probably an unfinished Fire Emblem stage)
Dummy (Apparently some other testing stage. The infamous Animal Crossing glitch item DUMMY was also a testing item)
IceTop (Believed to be an unfinished second Icicle Mountain stage)
10-2 (Possibly another Mario level, but never finished)