cop out vs slip through the cracks

cop out

verb
  • To avoid or shirk, either by failing to perform, or by performing in a grossly insufficient, negligent, or superficial manner. 

  • To plead guilty and ask for mercy. 

slip through the cracks

verb
  • To escape notice or lack sufficient attention. 

How often have the words cop out and slip through the cracks occurred in a corpus of books? (source: Google Ngram Viewer )