nit·pick 
(nĭtpĭk′)
(nĭtpĭk′)v. nit·picked, nit·pick·ing, nit·picks
v.intr.
To be concerned with or find fault with insignificant details. See Synonyms at quibble.
v.tr.
To criticize or find fault with (someone or something) in a petty way.
nitpick′er n.