Sad, sad discovery about the flagpole glitch...... but, before that, I can explain exactly what happenes.
First, when falling at full speed, the game checks to see if there is anything under the farthest right pixel of Mario's foot. Therefore, with perfect, perfect, PERFECT timing, you can make it so that the top pixel of the flag-block and the bottom-right pixel of Mario's foot become 1 pixel apart, side by side. Then, on the next frame Mario will appear to be inside the block. Holding left for 2-3 frames and then jumping allows Mario to get pushed inside the block, and jumping allows him to hit the extreme bottom, where no point value was thought possible, ignoring the flag lowering.
Now, the discoveries:
1) It is nearly, if not impossible to do it on all levels ending in 2 (except 8-2) due to lack of vertical speed after exiting the pipe, it is not possile to get the right positoning to enter the block, therefore enabling the trick un-useable.
2) Levels with unusual ending staircases also ruin the glitch, due to lack of horizontal spped and positioning.
Of course, someone could prove me wrong, and these are just discoveries in the first 3 levels of the warpless improv making.
EDIT: Adding something for lazy people upon request of andrewg
Well, in order to get the right positioning, you have to make it so that Mario lines himself up in the extreme top-right block (Mario's left foot is above the top-left of the block, vise-versa for the right) and then jump 3 frames. In order to do this, you need to jump from a certain place below, and in order to do that you have to jump from a place below, ect., ect....
It's hard to explain, so watch klmz's run closely at the stairs to fully understand.
EDIT AGAIN: HappyLee's proved me wrong >_>
See Youtube page (GIRfanaticTAS) for all runs and stuff.
