banana skin vs monkey wrench

banana skin

noun
  • A cause of difficulty, or potential embarrassment. 

  • The outer skin of a banana, especially once removed from the rest of the banana, which may be slippery if stepped on. 

monkey wrench

noun
  • An unexpected or troublesome problem, obstacle or dilemma. 

  • A wrench (spanner) with an adjustable jaw. 

How often have the words banana skin and monkey wrench occurred in a corpus of books? (source: Google Ngram Viewer )