The dead segment is either always on or always off.
All numbers look good.
The dead segment looks like a normal segment, it isn't highlighted or of different color. Finding the dead segment is your task.

I'm sorry, but the second and third examples don't make much sense.
|
|
and
_
|_
|_|

are both valid and clear numbers that both utilize the 'c' segment in their display, yet the 'c' segment is declared dead.
The same goes for:
_
_|
_|

(blank)

_
|
|

These are all valid and clear displays that don't utilize the 'f' segment yet the 'f' is declared dead.
In neither of these examples is the dead segment evident. Might I suggest that you either use different numbers or displays that clearly show a segment as dead or write the numbers that are supposed to be displayed to highlight what the actual issue is.
(If there is something obvious that I am missing, please point it out.)