漏網之魚
lòu wǎng zhī yú
Literally means, “fish escaping from the net.”
It can refer to a person who is lucky enough to escape punishment by law.
E.g.:
儘管Facebook、YouTube和Twitter已經部署了人工智慧軟體，並雇了成千上萬的內容管理員來刪除惡意或誤導性的帖子，但危險內容仍不可避免成為漏網之魚。
Though Facebook, YouTube and Twitter have deployed artificial intelligence software and hired thousands of content moderators to take down vile or misleading posts, dangerous content inevitably slips through.
(Source: https://cn.nytimes.com/china/20170526/google-beats-ke-in-go/zh-hant/dual/)
E.g.:
香港已經在竭盡全力地限制張德江到訪期間的抗議活動，但一些倡導民主的信息還是成為了漏網之魚。
The city has gone to great lengths to contain protests during Mr. Zhang’s visit, but pro-democracy messages have slipped through.