How to reproduce:

1. Enable additional languages in the input settings for your Operating System
- there should be a U.S. layout as well as a British layout (neither of them should be International)
2. Open a new Unity project
3. Create a new C# script named "InputTest" with the following code:

using UnityEngine;
using System.Collections;

public class InputTest : MonoBehaviour {

bool button;

void OnGUI()
{
GUILayout.Label ("Pressing button: " + button.ToString());
}

void Update ()
{
if (Input.GetButton ("Jump"))
button = true;
else
button = false;
}
}

4. Attach the script to anything in the scene
5. Switch to the British keyboard layout (http://www.goodtyping.com/teclatUKok.png layout for reference)
6. In Project Settings -> Input find the "Jump" axis and change the Positive Button from "space" to the apostrophe (')
7. Play the scene
- Make sure the layout is still British
8. Press the apostrophe
- Note how the text stays False
9. Press the backslash(\)/vertical bar(|) key (on a US keyboard) or the hash(#)/tilde(~) key (on a UK keyboard)
- Note how the text changes to True

The hash(#) button is unbindable. Setting the Positive Button in step 6 to "#" results in no button working for that Input (not even Shift+3). This is also the case on OSX.