This was logged a few (or more) months ago, but I could not reproduce it.
Please describe your sequence settings and media formats.
Happens with widescreen DV sequence preset and with the AVCHD 720p60 sequence preset (possibly others, couldn't say)
Just toggle the Zoom Level to 100% or 200%, make sure the Safe Margins are turned on. Resize the program panel to make it smaller and smaller, or contort it however you choose. Notice that the safe margins remain within view constantly, even when the "artboard" or "canvas" (to borrow Illustrator terms) is clearly well outside the boundaries of the program window.
Does that help?
The footage in all cases is AVCHD, 720p60.
I understand the symptoms, as well as the steps to get there. I just haven't been able to reproduce the issue, even with the guy I work with who found it.
I will keep looking.
It's quite frustrating.