In destroyed and incomplete meshes errors may occure.
In most cases undo will work, but in some cases even this step does not work.
If undo works: repair the mesh with some ,mesh repair-tools';
there are some fantastic plugins available.
if undo does not work, use the CLEAN-function from the pulldown menu > general functions (this tool is still faraway from a beta)
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In some cases the terrain must be restored manually.
There are some mighty tools available for mesh editing, edge correction an so on,
just visit the ruby archives (trimble, sketchucation etc.).
