Unity 4.6.3f1 bug or normal behaviour? MouseButtonDown cancelled/ ignored by key press

I'd love to download an old version of Unity and test this but I have got the sorriest internet and it'll take about 20 hours.

I have a huge problem with mouse clicks. Using the current latest version (4.6.3f1).

I have a simple little script attached to the camera in an empty project:

 #pragma strict
 
 function Update () 
 {
     if (!(Input.GetMouseButton(0)))
     {
         print ("Not down");
     }
 }

And as you might imagine, when I haven't got the left mouse button down it prints "Not down" into the console like crazy. It stops when I click and hold the mouse button.

However when I press ANY button on the keyboard while the mouse button is down it seems to ignore the fact I have the mouse button down and starts printing again.

What's worse is that when I hit a key on the keyboard and then click the mouse down within about a second of having pressed the key it totally ignores it.

Can anyone shed any light on whats going on?

Thankyou very very much,

Romano