Transporter psychosis was a fatal medical condition caused by a breakdown of neurochemicals during transport. A victim of transporter psychosis suffers from hallucinations, as well as other physical symptoms.
Transporter psychosis was officially diagnosed in 2209 on Delinia II, though there were reported cases as early as the mid 22nd century. It was eliminated by the invention of the multiplex pattern buffer in the 2310s. (TNG episode: "Realm of Fear"; ENT episode: "Daedalus")
In 2369, Lieutenant Reginald Barclay believed he was suffering from transporter psychosis after being infected by quasi-energy microbes while beaming from the USS Enterprise-D to the USS Yosemite. (TNG episode: "Realm of Fear")
In 2376, Commander Sonya Gomez also briefly considered whether she was suffering transporter psychosis, after encountering an alternate universe version of Kieran Duffy during transport. (SCE eBook: Lost Time)