Worse-Case-Scenario = is a win or lose situation but even if you lose, you still got an option to win after losing.
Example:
You boot up your PC, then a blue screen occurred, then PC can't recognize the hard drive. So, you removed the hard drive from its slot then put it back again. Now this is the worse case scenario, cause if the PC recognizes the hard drive, you win, but if not, you lose but you still got other options to win the situation, it's either you borrow or buy another hard drive, on that moment, you win.

Worst-Case-Scenario = is a win or lose situation, period. It has no secondary option if you win or lose, it is like, live or die.
Example:
A dam is about to overflow on a city and you are the only one who can stop it from happening. If you managed to stop it, you win, if not, millions will die. Now, this is a worst-case-scenario.