I had a number of areas in my house where the tape lifted at the joints. The drywall mud was apparently applied very thinly. I have this same problem with the ceiling.
A number of posts suggest sanding with a wet sponge. I tried this, but I'm having a little trouble with this approach. Whenever I sand it (with a sponge for sanding drywall), I get a nice smooth surface, but there is always an edge where the mud meets the painted surface. It appears that the damp sponge is softening the drywall mud enough that it loses it's adhesion and I can't get a smooth transition. The edge shows up when I primer and paint over the drywall.
I found that to get the best finish, I had to wet sand the area, let it dry, then follow up with dry sanding.
I'd like opinions on how to wet sand without getting the slight step where the wall transitions to the mud. Am I sanding with too wet a cloth? I tried with a dryer cloth, and it didn't sand very well.