Fault unreasonably found in chair sometimes (5,5)
I believe the answer is:
split hairs
'fault unreasonably' is the definition.
I can't explain the rest of the clue.
(Other definitions for split hairs that I've seen before include "a trichologist may deal with" , "cavil exceedingly" , "Worry about minutiae" , "Make pedantic distinctions" , "Make unnecessary points" .)