This is on Lubuntu 14.04. My system has 2 desktop users who may both be logged in. I use the "switch user" option on the Logout popup to switch between these users. I don't recall exactly how I created the 2nd user, maybe used "useradd -m". They do not have the same groups. user2 is logged in first.
Groups for user1 are user1 adm cdrom sudo dip plugdev lpadmin nopasswdlogin sambashare.
Groups for user2 are user2 nopasswdlogin.
When both are logged in and I insert a flash drive into a USB port, both users' desktops try to mount it. Various error and other messages and dialogs are the result, and the drive fails to mount. User2 will ask for administrator password while user1 gets a popup saying that an operation is pending. By switching back and forth between the users and canceling the dialogs, I can get the flash drive to mount.
Status changed to 'Confirmed' because the bug affects multiple users.