From the game Mega Man 3 for the NES.
After you have the Shadow Blade, but before you have the Rush Jet, go to any weapon power-up in any stage. Highlight the Shadow Blade in your inventory and press Right. Even though you did not yet earn the Rush Jet, due to a glitch in the inventory system, it will be highlighted. Now pick up the weapon power-up and you can use the Rush Jet.
Taken with permission from Flying Omelette.