jantar, there are a myriad of ways for a crew to show up for their mission/shift and not be on their A game.
Example:
Sergeant A has gotten an arse chewing or two during the past month for not being on time. The Captain has put him "on the sh** list"
Sergeant A has been feeling sick, but does not want to draw too much attention to himself by asking to go to the doctor.
The Captain has been keeping an eye on him.
He gets some medicine from (wife, friend, brother, cousin who runs a drug store / chemist's) and self medicates.
The stuff's a bit strong, so he's a bit foggy and is not as sharp as usual ...
Could it happen?
Yes.
And of course there are training issues, and more, in human factors cases ...
(And
this accident was often brought up when our flight surgeons discussed why not to self medicate ...)