Outlook freezing with emails that have links without http protocol block?

I've gotten two reports of Outlook freezing since changing the structure of an email I send to use image sources like:

//example.com/file.jpg

instead of:

http://example.com/file.jpg

Is this possible or is it a coincidence (2 reports out of 150k emails - but we've never had this reported before the change)?

I've had problems with links like this in Outlook when Internet Explorer is not the default browser. In my case the user had Chrome as the default browser and Outlook was hanging handing over the link to Chrome - it did eventually load the image, but it would take literally 5 minutes of being non-responsive.

Anyway, I would have thought that you really should be specifying the protocol in the hyperlink as best practice, but I'm not sure it will make much difference in this case