Maureen in Abbotsford BC has nail heads popping out of her walls a bit all over the house. This is unfortunately so common that the term "nail popping" is used to refer to nail heads showing up in drywall. The problem is that there is a space between the drywall and the stud, and when someone leans on the wall, the drywall moves back and the nail head moves forward, popping out its plaster cap. The space is usually caused by not pushing the drywall tight against the stud in the first place, something not always easy to do when there is fiberglass insulation pushing out during construction. It can also be caused or aggravated by building with wet studs and when the wood shrinks, the nail doesn't go deeper into the wood, hence the head ends up further from the stud.
The solution, as you can see in the photo, is to drive two nails, or better yet, two drywall screws, one on each side of the offending popped nail, about one inch away. Push the drywall hard against the stud with your free hand while doing this. Then once the drywall is in place, pound the offending nail in below the surface. Now you have to apply drywall compound over three holes rather than one, but at least this time it won't pop again.
Long nails in original construction tend to cause more popping than short nails. Half inch drywall should be attached with 1-1/4 inch nails or screws (only 3/4 inch into the wood). Longer nails are stuck deep into the wood and will not follow the shrinkage as well as short nails will. Screws follow shrinkage better than nails. Building with kiln dried wood that will not shrink and assuring that the drywall is snug against the stud before driving the fastener is the best way to avoid nail popping.