Nail pops are the small round bumps or cracks that appear on walls and ceilings as a fastener works loose and pushes the drywall compound out. They're cosmetic but annoying, and there's a right way to fix them so they stay fixed.
Why they happen
As lumber dries and the house settles and moves with the seasons, nails can back out slightly, popping the compound that covered them. They're most common in newer homes as the framing dries out.
Fix it for good
Don't just hammer the popped nail back in — it'll pop again. Instead, drive a drywall screw an inch or two above or below the popped fastener, into the same stud or joist, to firmly re-secure the drywall. Then drive the old nail slightly below the surface or remove it.
Patch and paint
Cover the screw and the old fastener spot with two or three thin coats of joint compound, feathering the edges, then sand smooth, prime, and paint to match. Done this way, the pop won't return.