Did you, Andrew?
I think you will find that when there is a VERY slow connection to CM (can happen for many many reasons - not necessarily due to your computer or CMs systems) as the page loads there are "place holder" icons that load prior to the actula final icons being displayed.
Normally a page will load so quickly that these types of place holder icons are never actually visible.
When I am using my phone as a hotspot for wi-fi to my laptop - I can experience the wierd boxes you mention appearing - where ever you would normally see a different icon.
The above is not necessarily the exact cause of the anomaly you experienced - but I have seen similar behavior to what you describe when I have a very slow connection on my computer.
The reference is only to show the icons you have seen - the actual problem being described at the link has nothing to do why it shows that way here on CM - as mentioned earlier, it is simply a slow connection that can cause the odd behavior, and is only temporary.