[rmrfrmrf]

The biggest potential problem is using a file/global-scoped "use strict" directive in a .js file that gets concatenated and minified with other files that are not strict mode compliant, thus unintentionally modifying the behavior of any code that's concatenated after the strict mode script.

This issue bit Amazon in the ass a while back and gave rise to JSHint checking for function-level strict mode declarations only and the general rule-of-thumb to not use strict mode in production code.