I've noticed that on the Parenting beta site, the notification bars slightly overlap the normal site header. This might be specific to Parenting but I think it might be a bug in the overall beta design.
This bug only appears at the third simultaneous notification. It looks correct for only one and two notification bars. I haven't seen four notifications so I can't tell how that would look.
I don't think that it's a browser quirk in itself, because it seems to work just fine for one and two notifications.
I'm using Firefox 9.0 on Windows XP, both US English.
At least on apple.SE there is no overlap even with three banners (I've upgraded to Ffox 10.0 in the meantime):