The Diablos Menu glitch is something that happens after defeating Diablos. The first time a menu would open, e.g. from pressing the menu button, entering a shop, or getting a forced menu opening (like renaming a character), the menu doesn't open and instead the screen fades to black and then instantly back to default. This can be used as a free menu buffer for step count purposes, but other beneficial uses have not been found.
This glitch doesn't exist in the PSX versions of the game, but is there for the PC and HD versions.