重箱の隅をつつく

[juubako-no-sumi-wo-tsutsuku]

Literal translation:

poke the corners of a multilayered box

Meaning:

nitpick

Another expression:

重箱の隅を楊枝でほじくる

[juubakono-sumi-wo-youji-de-hojikuru]

= 重箱の隅をつつく