Glitch dialog or glitch dialogue refers to the text displayed and the sounds played with invalid message box data in the Pokémon games.
Glitch dialog is normally undecipherable and is nonsense without rendering the text box data as pure code and looking into what it was meant to be. Glitch dialog may use glitch characters, that may be taken from other areas of the screen. 'M (00), 'M (FE) and 'M (FF), for example use glitch characters in their name.
Control characters such as the name of the player (hex:52 in Red/Blue) or the name of the rival (hex:53 in Red/Blue), or ROCKET (hex:5E in Red/Blue) may appear in glitch dialog.
In Generations I and II, when a hex:00 character appears in the middle of the text data, an error code will appear, in the format "(X) ERROR." (Red/Blue) or "(X) error." (Yellow) with everything after it not shown.
In Pokémon Gold and Silver error codes were untranslated and appear as "(x)Df-"
Forcing glitch dialog in text boxes
Like how most Generation I glitch Pokémon have glitch dialog as their name, there are ways to force glitch dialog in text boxes.
It is very common through hacking, but there are glitches to force glitch dialog as well, including (note this list is not meant to be exhaustive):
Example text box ID matching method
In Pokémon Red, Blue and Yellow, the player can use the Trainer escape glitch's text box ID matching trick to get glitch dialog when they return to the place where the Trainer they escaped from was.
For example, in Pokémon Yellow one can escape from the long-range Trainer in Route 6 (north of Vermilion City), battle a Trainer on Route 11, then go to the Vermilion City PokéMart, check the shelves to bring up the "Wow! Tons of Pokémon stuff!" message (hex:44), and return to Route 6 to have glitch dialog loaded.
The same method can be used in Pokémon Red and Blue, but the shelves load value hex:42. In this case, returning to Route 6 loads three glitch characters followed by "66 ERROR".