__ on the side of caution (4)
(Other definitions for err that I've seen before include "do something you shouldn't" , "Drop a clanger" , "Boob" , "Leave the straight and narrow" , "Deviate" .)
(Other definitions for errs that I've seen before include "Slips up" , "Goes wrong" , "Sins" , "Boobs" , "Goes astray" .)