The Man From Nowhere
It Wandered In From the Wastes

Occurs when a more minor character or plotline is suddenly dropped for no apparent reason, without any real explanation for what happens, and without any sort of resolution to the character or plot.
Ex.
Fallout 2 - We never do rescue Sulik's sister or find any trace of the slaver group whom kidnapped her.
Fallout: New Vegas: If you convince Follows-Chalk to leave tribal life to see civilization for himself. If you do he's said to leave his tribe at Dead Horse Point and is never seen nor heard again.
Fallout 4 - If siding with The Minutemen, The Railroad, or BoS (without recruiting Dr. Li) we never do find out what becomes of Dr. Madison Li when (and if at all) we sound the evacuation alarm before blowing up The Institute. A major character from 3 and one of the few to return from the Capital Wasteland disappears and is never seen or heard from again.
Any more examples?
Ex.
Fallout 2 - We never do rescue Sulik's sister or find any trace of the slaver group whom kidnapped her.
Fallout: New Vegas: If you convince Follows-Chalk to leave tribal life to see civilization for himself. If you do he's said to leave his tribe at Dead Horse Point and is never seen nor heard again.
Fallout 4 - If siding with The Minutemen, The Railroad, or BoS (without recruiting Dr. Li) we never do find out what becomes of Dr. Madison Li when (and if at all) we sound the evacuation alarm before blowing up The Institute. A major character from 3 and one of the few to return from the Capital Wasteland disappears and is never seen or heard from again.
Any more examples?