To further expand on what the previous posters mentioned...My home was vacant for 4 years before I purchased, therefore, the lack of constant temperature and humidity really accelerated the popped nail situation. Here's what I did to several thousand (believe it or not) popped nails. First, using a utility knife, cut a circle around the bump to expose the nail head. If it is already exposed skip this step. Don't cut deep, just enough to remove the paint and spackle over the head. Then use the ball end of a ballpeen hammer to drive the nail in about 1/2 an inch. Gently tap the edges of the dimple to minimize the cracking of paint. Now screw a 6 x 1 5/8" COARSE drywall screw along the head of the nail so you overlap the nail head. You can now spackle over the dimple. I've been successfull with 3-4 coats...but I'm a perfectionest and perhaps there is a spackle that shrinks less. Oh well...I have not had ONE nail come back out in six years and the walls/ceilings look great. Not fun but a good work out. Funny thing is that when I go to someone's house I can spot nail problems a from mile away. Hope this helps.