From Glitch City Laboratories
The message "No windows avail-able for popping." as it can appear on Pokémon Gold and Silver.
The phrase "No windows avail-able for popping." exists within the coding of Pokémon Gold, Pokémon Silver and Pokémon Crystal, however it is normally never seen through normal gameplay without the use of a glitch or a game-altering device.
As the phrase suggests, it will be displayed when a byte within the game attempts to bring up a window, but the data corresponding with that window is invalid or undefined. Thus, it is presumably an error handler so that the game does not crash when such an event happens.
- The message can be seen when forcing HM06 outside of the TM/HM pocket through a gameshark code such as 01F8B8D5, when used the game would read invalid data for ordinary items and bring up a menu as if the player used a PC. If the player decides to close this window the "No windows avail-able for popping" message will be displayed and the game will crash.
- In Pokémon Crystal, a few Glitch Dolls have the potential of attempting to bring up an undefined dialog box other than the default when checked, thus bringing up the "No windows avail-able for popping." message
- The message can appear (rarely) when using a walk through walls code to move out of bounds. 
 - The "No windows avai-able for popping." message, as documented on Glitch City Laboratories forums.