Since FDM printing involves melting polymers and feeding them through a very small hole, Nozzle clogs are one of the most common and one of the most annoying problems to occur with a 3D printer. Repair can be as simple as pushing a hard alan wrench through the barrel, some can be as time consuming as dissambling the carriage and torching the material out.
As mentioned in our Cooling chapter, a barrel fan is needed for almost all FDM printers to succeed without heat creep causing a nozzle clog. While feeding material out of a hot nozzle, the heat will often creep up the filament. This causes the filament to expand inside the barrel, often creating a lump. This lump will cause a clog and will make the removal of the filament from the barrel near impossible without disassembly.