I had to ban someone tonight (omg my server was active) for building stuff under ground. I'm sure valve's aware of it, but this means they haven't fixed that underground bug across the board.. so anyone with a lot of displacement brushes beware.
There's a place outside the second red spawn on the right hand side where an engineer can place a teleporter exit outside the fence. And transport himself out of the map. Then through some running around he can get below the map and start building.
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