This episode of Wednesday Wiki is dedicated to Phantosmia. Find it here.
Phantosmia is a condition where a patient experiences "olfactory hallucinations" where they smell things that are not there. The smells patients experience are typically nasty smells like rotting flesh (creepy much?).
This condition has several reported causes including seizures, Alzheimer's Disease, brain trauma, schizophrenia, and sinus infection. So if you like your sniffer to convey reality instead of zombie flesh, avoid strobe lights, aging, hitting your head, listening to the voices in your head, and nasal congestion. Yikes.
The episodes of Phantosmia start out infrequently, but can evolve into a daily occurrence. Thankfully, they seem to only last for a couple minutes then go away. Unfortunately, the only treatment seems to be surgery called transnasal excision. That sounds almost as bad as smelling zombie rot. Freaky, freaky, freaky!