nitpicker (n.)

also nit-picker, "pedantic critic," by 1951, perhaps 1946, a figurative use, said to be originally military jargon; see nit (n.) + pick (v.).

Two long-time Pentagon stand-bys are fly-speckers and nit-pickers. The first of these nouns refers to people whose sole occupation seems to be studying papers in the hope of finding flaws in the writing, rather than making any effort to improve the thought or meaning; nit-pickers are those who quarrel with trivialities of expression and meaning, but who usually end up without making concrete or justified suggestions for improvement. [Collier's, Nov. 24, 1951]