Making a customized world, gravel and dirt on the ores screen go up to 256, but clicking on it only allows you to go up to 255.
This's been in the game since custom worlds were added.
This is useless to have it go to "Y=256" when you can only build up to Y=255.
This ticket has been marked as a duplicate of MC-69990, in favor of the more general or better information provided.