MAX_QUEUE_READERS and Activated Procedures

If I set MAX_QUEUE_READERS =1 for a given queue, am I guaranteed to only ever have one instance of the associated activated stored procedure running at any time? I've run into an issue where it appears that more than one instance of the activated stored procedure is running even though MAX_QUEUE_READERS =1. The code was not designed to run multiple instances concurrently.

Thanks in advance.