There are many things that can go wrong when winding pickups. Here are some that can be problems for beginner and experienced winders. Here are a few hand winding problems. Hand winding is often called scatter winding.

When winding with to much tension the magnet wire can stretch causing the coil to have a higher DC resistance.
Winding with to much tension can cause the top and bottom flatwork or molded bobbin to flair causing uneven coils.
Winding a coil to fast can cause the coil to become spongy and magnet wire can become stretched causing the insulation to crack.
Winding a coil to fast can also cause loops or snags from the edge of the flatwork or bobbin. The edges need to be smooth and parallel with the magnet wire.
Magnet wire that has cracked insulation that exposes the bare copper wire can eventually oxidize causing the turns to short out that can lower the DC resistance or cause the pickup to fail.
Coils wound with inconsistent or not enough tension can become spongy and cause the DC resistance in pickups to vary considerably.