This bug seems to happen at random, although I can see it at least once every time I reload the page. For me, I can see this every time I go onto my profile and look at my friends, or when I am scrolling through game instances. One area that I saw it particularly often was in the Adopt Me game instances, and as I was scrolling through it multiple failed to load.
I have been noticing this happening for most of the day today, and I have confirmed with other individuals that I am not the only one experiencing this.
I am currently using the latest version of Chrome on Windows 10, and I have all my extensions disabled.
Can confirm this rare occurrence. Sometimes avatar images are missing on certain users. I can also include that the avatar editor has some issues with previewing. It sometimes fails to give you the correct avatar drawn.
