(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.