straight from klov:
Cheats, Tricks and Bugs
There is a very risky maneuver that can lead to either disaster or a powerful comeback. When Sinistar has you in its grasp, a spare shot that is floating in space can kill you, of course taking a life. The unfortunate thing is that when Sinistar's mouth closes, another life is taken, resulting in the loss of two lives. Bear in mind that the Warrior ships will not fire at you when you are in Sinistar's grasp, so a floating shot is the only thing that can make this occur. However, if this happens to you when you are down to your last life, the loss of two lives will reduce your number of lives to -1. This will give you 255 lives.
on a side note, does mame have the easier difficulty settings from before they made it more difficult?