come across vs fall upon

come across

verb
  • Used other than figuratively or idiomatically: see come, across. 

  • To find, usually by accident. 

  • To produce what was desired; come up with the goods. 

  • To give an appearance or impression; to project a certain image. 

  • To change sides; to cross over to work for the opposition. 

fall upon

verb
  • to experience; to suffer. 

  • to occur at some particular point in time. 

  • to set upon; to attack suddenly 

How often have the words come across and fall upon occurred in a corpus of books? (source: Google Ngram Viewer )