I was perplexed for days with a weird scenario. If I maximized my MDI child form, after browsing through a few forms, it would start displaying multiple control boxes on top.
I searched for a solution for a while, but no luck. Found this article, which describes almost the same problem but no response: http://www.tech-archive.net/Archive/DotNet/microsoft.public.dotnet.general/2004-08/1063.html. Maybe this guy will come across this post and it might help him (after 4 years! =)
Anyway, the fix is actually very simple, and turned out to be an oversight. The "Main Menu Strip" property on the parent form needs to be set to the menu strip. If it's not, then you will get the wierd behaviour, which is probably a bug in Visual Studio.
On the subjet of menu strips, if you don't want your mdi child menu items to "merge" with the mdi parent menu strip, just set the "AllowMerge" property to False.