break-off vs stumbling block

break-off

noun
  • A break, discontinuance, or interruption 

  • That which breaks off, or is broken off 

  • The first shot in a game of snooker or other cue-game 

verb
  • To play the first shot in a game of snooker 

stumbling block

noun
  • A hindrance, obstacle or impediment; an opportunity for error. 

How often have the words break-off and stumbling block occurred in a corpus of books? (source: Google Ngram Viewer )