What are some of the worst mistakes people make after having a cast removed from a fracture injury? ~

Immediately expect the limb to work exactly the same way it did before. It took several weeks of physiotherapy to get something approximating the same range of movement in my thumb (and I'll never get it back fully, Arthur has the joint in permanent thrall.)

Being too nervous to use the affected limb and delaying their recovery.

probably injuring it again