Where the scene-shifters may be making mistakes (5)
I believe the answer is:
slips
I believe this is a double definition.
'where the scene-shifters may be' is the first definition.
(the slips are the sides of a stage)
'mistakes' is the second definition.
(slip is a kind of mistake)
'making' acts as a link.
(Other definitions for slips that I've seen before include "errors" , "They try to catch you" .)