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When distributing a map or mod, care must be taken to ensure that any custom content (sounds, materials, textures, models, etc.) is also distributed, or else players will have an unintended experience.
- Missing materials/textures show up as a black-and-magenta checkerboard.
- Missing models show up as red 3D text saying "Error" (in Portal 2 the 3D Error text is a light blue)
- Missing sounds are absent (in Portal 2, a missing sound is indicated by Dr Kliener saying "Oh, fiddlesticks.")
- Missing particles (at least in Portal 2) appear as a spray of red X's.
If the missing contents shipped with the Source SDK, and are not custom creations, it is likely a problem with your game configuration. See Incorrect Hammer configuration for more information.
One very common problem is when users see a black-and-magenta checkerboard "sheen" or reflection on surfaces. This is caused by a missing texture, generally due to missing cubemaps, which must be "built" in-game prior to distributing the map.