When a material's shader fails to load, its surface turns a vivid magenta. Builders call it the error shader. We call it what it is: a hazardous material, installed in good faith years ago, that is now in every wall.
It is most often found in worlds built before a platform update, in avatars uploaded from an old project, and in the one chair a friend imported "just to test". Magenta in good condition presents very little risk. Most incidents begin with someone opening the project to fix it.