What is the meaning of sub pending?

Seems to be a bug, I think it happens most frequently when trying to subscribe to a community on a busy instance. It doesn't seem to affect functionality though, I see new posts, vote and comment, etc. Sometimes if you refresh the page or try again it goes away and says subscribed, but I wouldn't worry, doesn't seem to change anything right now. Just assume it means subscribed. I think for this community right now it says "sub pending" for me actually.