Once I refactored some stringly typed C# code to use an enum with values Prod and Test instead of the two strings "Prod" and "Test".

I was relying on expressions like

  mode == "Test"

to fail to type-check to spot all the places that I needed to update.

Sadly some parts of the codebase weren't very idiomatic and had some

  "Test".Equals(mode)

This caused a bit of involuntary testing in production and thought me not to trust anything while refactoring.