Where to find warp zone #1 hidden in
level 1-2
You will find the first Warp Zone
located at the end of level 1-2. In
order to reach it, you will need to
ride the previous elevator to the top
and then jump onto the brick ceiling.
Run all the way to your right and you
will find the Warp Zone. Enter the
corresponding Pipe to skip to world’s
2, 3 or 4.
Where to find warp zone #2 hidden in
level 4-2
This level contains two Warp Zones,
one to level 5 located at the end of
the level, and this Warp Zone that
takes you to levels 6, 7 and 8 located
in the middle of the level. To find
this Warp Zone look for a high
three-brick-long platform that is too
high for you to reach, located across
from the first elevator platform.
There are some invisible blocks that
you will need to make appear in order
to reach the far left block, which
contains a vine. You need to hit the
invisible blocks from below to create
a staircase allowing you to reach the
Vine. To do this, make the first two
lower blocks on the righter side
appear. They will start to create a
staircase. Then hop on them and break
the two bricks above. If you haven’t
already activated the vine, try to do
so now, and then make the following
invisible blocks appear if you need
them to reach the vine. Climb the vine
and you’ll reach the Warp Zone that
can take you to worlds 6, 7 or 8
depending on which pipe you enter.
Where to find warp zone #3 hidden in
level 4-2
To reach the final Warp Zone (note
that you can only take one Warp Zone
since both are on the same level), you
will need to reach the ceiling from an
elevator platform and then run all the
way across it to the right, just like
you did in level 1-2. This Warp Zone
will take you to World 5.
Best Answer
Generally when debugging a game under development, a programmer puts in various ways to skip ahead to later levels (such as key combos) that don't always make it into the final released product. This means warp zones were probably not for testing purposes.
I suspect they were just fun and interesting secrets.