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Two people I've hired to build houses in a commercial grid I'm developing have had the same problem: invisible force fields (for lack of a better term!) at certain locations in their houses. Both are experienced mesh builders in Second Life who have not encountered the problem before. It's impossible to walk through these "force fields".
I'm using OpenSim 0.8.0 Development and have tried it with Firestorm and Singularity viewers on OpenSUSE Linux 13.1 and Windows Server 2008.
One house was originally built in Second Life; the other was built using SketchUp and imported directly into OpenSim.
I don't have any mesh experience myself yet; I'm hoping someone here will have a suggestion I can pass on to the builders for how to fix their houses.