Let's say you have:
- a doc with _id foo/bar
- a show function named baz
- a update function named baz
Then _show/baz/foo/bar is valid but _update/baz/foo/bar is not. Only _update/baz/foo%2Fbar works.
This is particularly annoying when you want to rewrite /something/* to _update/baz/foo/* (rewriting /something/* to _show/baz/foo/* works perfectly).