The Seafoam Islands glitch text boxes (Japanese: ふたごじまのぢらい, Seafoam Islands no Jirai) are unintended 'sign texts' that may be found in Seafoam Islands in Pokémon Red, Green, and Pokémon Blue (Japanese).
In Japanese Yellow and the localizations the hidden text boxes were removed completely.
Right island entrance
The right text box in Japanese Blue.
In Pokémon Red and Green, pressing A on a certain position in the Seafoam Islands' right island entrance causes an empty text box to appear then immediately close itself. In Japanese Blue a blank text box appears too but in this case no audio plays anymore and the game hangs; you can't close the box. Itemfinder does not detect an item here in both versions.
After the message has been closed, the game will take a longer time at scrolling through the Pokédex with right or left on the d-pad, displaying the options on the 'summary/switch/cancel' box and at showing your Pokémon's status, due to the byte D2D7 being kept at 03, when it's normally kept at 01 for most of the time, only changing to 03 for a fraction of a second when you view certain messages. The 'slowdown' can be fixed by pressing start and going into the options, then exiting. D2D7 is not to be confused with D2D4, the Japanese options address.
Left island entrance
The left text box in Japanese Blue.
The left entrance of Seafoam Islands also has its own tile with text box data, as documented by ShaneM on the DKC-Atlas forums.
Talking to the left entrance sign on Red/Green resets the game, while talking to it in Japanese Blue causes glitch dialog to appear including a lot of '...', and the music will change to the percussion of Pokémon Mansion.