The most common is probably a printer which has a blinking data light and looks fine but is not printing anything. This can usually be fixed by just power cycling the machine. Sometimes the printer will be stuck with its data light on solid. This can be cleared by a power cycle usually as well. Occasionally a printer will appear frozen when in fact it is merely offline.
Users can be really silly sometimes.
These are cleared by a power cycle. This method will usually fix ANY queue which is stuck. This page by plague .