This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Frequency
With John's dream (as the timeline changes again), the audience is led to believe that it's a remnant memory of his father's funeral before the changes in the past. Then we find out it was his mother's funeral.
In a rather frightening case of What Might Have Been, Frank gets caught by Shepard (the Nightingale killer) while trying to protect the nurses and almost becomes a victim himself. It's only by the grace of two hippies interrupting Shepard that he was spared.
Shepard find his way to Frank and Julia's room...while Julia thinks he's Frank.
Shepard's hand getting shot off in the past and thus necrotizing and withering away to nothing in the present.
While we know what's really going on the whole time, the first contact through the radio is quite disturbing from Frank's point of view, as a strange man suddenly contacts him, is unusually knowledgeable about his family, and finally states that he can see how Frank just burned his desk.