When is it acceptable to not trim a user input string?

Can someone give me a real-world scenario of a method/function with a string argument which came from user input (e.g. form field, parsed data from file, etc.) where leading or trailing spaces SHOULD NOT have been trimmed? I can't ever recall such a situation for myself. EDIT: Mind you, I didn't say trimming any whitespace. I said trimming leading or trailing (only) spaces (or whitespace).