Here you can see a spark plug with heavy deposits. This can be caused by poor fuel quality, high oil consumption from a mechanically-worn engine or burning of coolant from damaged cylinder head seals and can promote glow ignitions (the deposits glow after).
An insulator break, as is visible in this image, can lead to engine damage. The cause of such insulator breakage is the use of the wrong torque or the spark plugs were dropped on a hard surface (e.g. workshop floor) before installation.
The middle and earth electrodes have melted together on this spark plug That happens if the spark plug overheats. In this case, it is also possible that the piston could melt. The cause could be the selection of the wrong spark plug (incorrect heat rating) or a malfunction of the engine (pulsatory combustion or glow ignition).
Here you can see a spark plug clogged with carbon deposits. Carbon deposits appear if the spark plug is frequently operated below its self-cleaning temperature (450 °C) - for example, when only short distances are driven or an incorrect heat rating (too cold) was selected.