I'm sure this is a noobie problem, but... I decided to model very simple coffin so I could train texturing. I tried to weld some faces in uvw unwrap editor, but there are some "frozen" vertices. I've painted the problem area in red. I cant select the vertices from that polygon but I can select it as a face or edges. All…
I think this is because sometimes when planar mapping, the W value gets shot off to a crazy amount which makes it unselectable because it's so "far away" (even though it looks fine in UV view). You might be able to fix it by re-mapping the offending polygon, or it might be worth trying to change your view from UV to VW or…