Yanking out old, rusted nails is no fun, but the task becomes even more difficult when a nail head pops off. Here's an old carpentry trick that can be used to remove any headless nail, without damaging the board.
First, slip the hammer claw
around the exposed nail shaft. Then take a pair of locking pliers and tightly clamp it onto the nail as close as possible to the hammer claw. Slowly pull back on the hammer handle until the nail starts to pull out. Stop, reposition the hammer claw and pliers farther down on the nail shaft and repeat.
To gain a little extra leverage when pulling out large galvanized or ring-shank nails, replace the hammer with a long pry bar.