The phrase "worst nightmare" means the most unpleasant or frightening thing or situation a person can imagine. It refers to something that someone fears or dreads the most. Full definition
This can be worst nightmare of woman's life. (stylecraze.com)
So there's no question that it's every pet parent's worst nightmare for a pet to go missing. (onegreenplanet.org)
If recruiters could show you a visual of their darkest, worst nightmares when it comes to sourcing qualified candidates, functional resumes would be it. (thejobnetwork.com)