An interesting thing I found in Palm Tree Paradise...

[CM30]

I disabled layer 0 in this Wario Land 4 level, and look what I found:

Yes, those are the debug level tiles underneath the grass ones. Was this level in development before the graphics were finalised?

 

[ChanceTime]

Huh, must've been! Did you find anything of the sort in levels like Hall of Hieroglyphs? Maybe this could shed some light on what order the levels were made in if it appears in more.

 

[CM30]

Nothing in the Hall of Hieroglyphs. Just in this one room in Palm Tree Paradise.