Interesting Facebook glitch

Yesterday, I was randomly chatting on Facebook and out of nowhere, two icons appeared next to each message in my FB chat:

One icon looked like an arrow and the tooltip said “View message FBTrace” and the other one looked like a bug and said “Report a bug” – or similar.

When I clicked the trace icon, it took me to the following URL, but the page never loaded:*msg_id_num*

The icons were gone when I refreshed the page. I will probably never know what exactly happened, but I assume that this was a bug and I wasn’t ever supposed to see it, since it is an internal feature.