If you are receiving the message,"Your Pandora Account is currently playing on another device,"or"Someone else is listening,"then more than one device is streaming from your account at the same time.
While your Pandora account can be logged into as many devices as you like, they can't be playing at the same time. Regardless of your subscription level, the revenue from one Pandora account only covers the cost of one stream from one device. As long as you press pause on one device before you listen to Pandora on another, you should be fine.
If you believe this error message is popping up due to an unauthorized user and if you have access to a computer, you can take some steps to help protect your account by resetting your password and logging out of all other devices.