Headlight bulb(s) flicker, work intermittently, or repeatedly burn out.
Loose electrical connection. With age, vibration, and corrosion, electrical connections can weaken. In the pictured example, a loose fuse connection caused intermittent power to the headlights. Also, resistance created by the unintended gap caused excessive heat, ultimately melting the fuse and connector. The fuse failed to perform as designed and continued to supply power because the electrical draw remained within the amperage limit of the fuse.
The location of this fuse had to be rewired adjacent to where its original position was in the fuse box (most fuse boxes contain extra locations for additional fuses). The wires were properly attached and secured.