Since I couldn’t find any valuable information online about this rather surprising bug, I decided to sketch this tiny blog post to shed some light on it.

How it happened

Innocently, I tried to join a Discord server whose invite has expired. I alt-tabbed out of Discord to explore the rest of the website I’ve been directed to. Shortly after, after bringing the Discord window up, a server join popup has opened for the Discord “Undefined’s lounge” server. At first I was surprised and confused. Then, everything clicked in my head.

The vanity URL of the Discord server in question is https://discord.gg/undefined, meaning that poor quality code on Discord client’s side probably tried to concatenate an unset variable to the Discord invite.

It appears that I am not the first person to stumble upon this bug - it has been talked about on the Discord subreddit before, with the most recent comment being two weeks old.