I have two HTML panes in the same pane, both maximized so I can seem them at the same time.  Often as I am moving around and clicking through items I will notice that one HTML pane just shows a number on it, instead of a number and item title.  I'll have to click around through several items and then the title on the HTML pane suddenly shows up again.

 

I also had the calendar stacked in a pane once, and on that I'd notice that instead of saying "calendar" on the tab, it would be a number.  Clicking on the calendar would cause it to load, and the tab name would change to calendar.

 

Thanks