The Save A button held glitch is a very simple natural glitch in Pokémon Red, Blue, Yellow.
The player simply needs to save their game, and have the A button held when the "[PLAYER] saved the game !" text box disappears.
Hidden characters glitch
The text box and save menu will close, but not the START menu. Thus, the characters hidden by the save menu won't reappear until the player releases the A button.