This is driving me nuts. I have a scanned OBJ file ~4M faces and I'm trying to clean it up by deleting extra geometry that isn't part of the area I care about.
I'm using edit mode > box select > face and edit > box select > vertex. Both methods have this problem:
The selection doesn't get deleted, only some of it. I don't know why or how, but I just want the selection to get 100% gone.