land mine vs roadblock

land mine

noun
  • A critical subject or situation with concealed or unpredictable implications. 

  • A piece of exercise device consisting of a pivoting tube attached to the ground to which a barbell can be fitted to lift it off the ground. 

  • A mine that is placed on land and designed to explode when stepped upon or touched. 

roadblock

noun
  • An obstacle or impediment. 

  • Something that blocks or obstructs a road. 

verb
  • To prevent, hinder. 

How often have the words land mine and roadblock occurred in a corpus of books? (source: Google Ngram Viewer )