Not sure what is going on, but lately I seem to get a lot of invalid solid
errors whilst clipping. First I thought it was due to RMF format bug and/or clipping off-grid stuff, but now it's happening on normal solids using VMFs?Texture-Properties-Brush-Selection Suggestion
Maybe when you close Texture Properties dialog, have the brush(es) of whose faces you had selected, selected Clip-Outside-Edges-By-Default Suggestion
When clipping, maybe have Clip brush choose the outside edges of the polygon everytime, since 95% of the time that's what you're doing.