Tell me you can see that

Hallucinations has a lot of different causes, plot possibilities and emotional consequences for the whumpee.

It could be related to a trauma (like PTSD), the effect of some kind of drug or poison, a manifestation of their guilt, even a disease with neural symptoms.

There's also the possibility that the whumpee is being haunted, and just doesn't know it yet.

TV Shows
Arrow


 * 1x06: (flashback) Starving inside a cave, Oliver Queen (Stephen Amell) starts seeing his dead father, who gives him the choice of a quick death, handing him a gun.

The Haunting of Hill House


 * Well, this is a ghost story. All the members of the Craine family sees things that others (most of the time) don't. Each one has their own haunting, and all the emotional scars that this brings.
 * 1x07: Luke (Oliver Jackson-Cohen) is grabbed by a ghost and almost pulled into a grave. Steve helps him get away from it, but of course he doesn't believe on Luke.

Transplant


 * Through out several episodes, Bashir Hamed (Hamza Haq) is shown having these lapses, seeing things and sometimes losing gaps of time because of it. Eventually we learn that these "visions" are related to a traumatic experience during the war in Syria.

Travelers


 * 1x07: The whole team experiences some kind of hallucinations, as an after effect of the anti-toxin from the previous episode.