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, 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.