Breaktexture, also simply known as the texture bug, is where the style is set to something (such as "../../favicon.png?", or "undefined.png?") that invalidates all other textures, and renders all style-based textures as a single image.
Anatomy[]
The breaktexture sequences are directories to specific files in the /textures/ page. When they are linking to a file outside of the /textures/ page, they have to use "../" to go to a parent directory.
The ? invalidates the textures, as the texture is loaded like this: style/block/block.png
If the style piece has a ? at the end, it will pretty much ignore the /block/block.png part. (like how http://tileworld.xyz/docs?example is the same page as tileworld.xyz/docs)