Apparently it's possible to receive a guild invite while you are already in a guild.

I wasn't interested in testing whether I could accept it, since it happened on live, but this strikes me as something that should be checked before the invitation is sent and the person sending the invitation should be informed the target is in a guild.

You cannot accept it, it tells you to leave your current guild if you try.

This is a very serious issue and needs to be addressed ASAP.

BS. It's not a serious issue.

Another reason for the sarcasm tags :P

Ya. ;)

This is not a bug. Like tarenty said, it will tell you to leave your current guild if you try to accept it.
There's nothing wrong with inviting someone who's already in a guild. What if they want to change guilds? The inviter shouldn't have to wait for the invitee to leave their guild before sending the invitation.