The wood frame of an interior hollow core door isn't thick, and if you put pressure on it by, for example, slamming it or leaning on the knob while it's open, the hinge screws can split the wood. When this happens, the door leans and may not work properly. It looks like a major headache to fix, but it isn't, although you do have to take down the door. In fact, popping the hinge pins may be the most difficult part of the job, especially if the hinges are covered with paint.
Tap a nail through the hole in the bottom of each of the door hinges to free the hinge pin. Once it pops loose, grip it with pliers and pull it out. If the hinge has been painted, cut the paint around the head of each pin with a utility knife before tapping it. Keep the door closed while you're removing the pins.
Open the door, pull it off the hinges and lay it on a flat surface, such as a pair of sawhorses. Unscrew and remove the hinges with a Phillips screwdriver.
Work the head of a flat-head screwdriver into the split to widen it. Dab carpenter's glue onto the wood on one side of the split with a toothpick. If the split is wide enough, you can squeeze glue in directly from the tube or bottle.
Remove the screwdriver and clamp the wood together with a C-clamp. Put a small piece of cardboard on each side of the door to prevent the clamp from denting the wood. Wipe off all the glue that oozes out of the split with a damp rag, then fill the screw holes with epoxy wood filler.
Leave the door overnight, then sand the wood flat with 120-grit sandpaper. Drill a 1/8-inch pilot hole through each hole you patched, then replace the hinge, screw it on and replace the door.