bright spot vs knock-on effect

bright spot

noun
  • A good part of something that is otherwise fairly bad or unpleasant. 

  • Used other than figuratively or idiomatically: see bright, spot. 

knock-on effect

noun
  • A secondary, often unintended effect; a repercussion, chain reaction. 

How often have the words bright spot and knock-on effect occurred in a corpus of books? (source: Google Ngram Viewer )