Simultaneous procedures?

My tab panel procedure works well, except when I have an open active object on the form. When I have an open active object and click to change the tab, the procedure attached to the active object (let’s call it Procedure1) is started and the main tab panel procedure (let’s call it Procedure2) begins to run (as evidenced by an embedded debugging message) before Procedure1 finishes. It appears that Procedure2 is interrupted or terminated by Procedure1. I am probably doing something wrong. Has anyone seen this strange behavior? What are the rules two simultaneous procedures?