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airport_water_lowend.vmt

When you're on a low shader, the game will try to load liquids/airport_water_lowend.vmt as a fallback water material if a campaign tries to load liquid/ruralwater_drainage_pretty.vmt, but such material does not exist within vanilla install of L4D2.

This is a copy of ruralwater_drainage_pretty.vmt as airport_water_lowend.vmt without the lines that call for fallback material.

This happens most notably on Death Aboard, or rather I only care about that map.