IDMUS requests invalid music
When using the idmus cheat code certain track selections refer to music from the wrong game. This is especially critical when playing Doom, as the cheat was initially introduced only for Doom II (in the initial release version 1.666) without being disabled for Doom.
If the cheat is attempted during a game of Doom versions 1.666 to 1.9, it will quit to DOS because of a missing Doom II music lump unless idmus00 is typed, which will call D_INTROA, which is in Doom. The same error will occur when typing idmus00 in Doom II, on the other hand.