Yeah, I get those a lot too. I've also (recently) been getting an error with the insert edge-loop tool. When I click and drag in the surface, the preview edge doesn't move but the edge is inserted wherever my cursor is. So I am still able to place them where I want. I just can't preview them live on the mesh.
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"Your weapons are no match for ours! People of Mars, surrender!"
"Um, this isn't Mars. This is Earth."
"Earth? Earth-with-nuclear-weapons Earth?"
"Yes."
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