I have sometimes had a git branch called “tests” because I was adding tests or working on tests. The problem is that many projects have a “tests” directory so if you have a “test” branch, you get this:

$ git diff tests
fatal: ambiguous argument 'tests': both revision and filename
Use '--' to separate paths from revisions, like this:
'git  [...] -- [...]'

You can of course use git diff -- tests and it will work, but it’s easier to just not name a branch “tests” in the first place.