(I was just about to write about this:)
As a note:
jumping over the
flag pole in 3-3 (or was it 3-2) only lets you
run forever. It does
NOT allow you to get into
minus world. That is simply another common
bug. (Also, small firey looking
Mario is common). I have only heard of the jumping
backwards in
world 1-2 reproducing it. I don't know if jumping out of the water is
possible.
Minusworld is most certainly a bug, however. Yerricde gives an excellent description of why it is in the writeup below this one.
Update, i decided to give the minus world trick the old sporting try, and I got it to work under an emulator. However, I went down the second pipe and ended up in world 5. Quite a nice "feature" if you ask me.