Quite a bit, if you're Gtk#.
I've implemented the popup user menu in WikiBench.MediaWikiIntegration, along with the extension points for other addins to insert their own menu items into the menu. They can either provide a type extension using a class that derives Gtk.MenuItem, or a custom extension node provided by MediaWikiIntegration to add a separator. It was working perfectly... well, almost.
If you are using the Clearlooks GTK+ engine then you know that when you hover over a menu item, the text color changes from black to white. Lots of other themes do this too. Well, the text on one of the menu items in my popup menu was not, and it happened to be a menu item provided by an addin. So, naturally, I start poking at this phenomenon.
Is it because the class I'm instantiating is in another assembly? Nope.
What about the fact that it's instantiated using reflection? Nope again.
So I gave up and went to bed. Today I started digging around a bit more, and finally gave up again.
Later I decided to refactor a few classes, and one of the ones on the list was the menu item that was acting funky. The class was named BlacklistUserMenu, and since the other classes I'd defined were named things like TalkMenuItem and BlockLogMenuItem, I renamed it to BlacklistMenuItem for consistency. Then I compiled and ran the program to make sure my changes worked.
They did. In fact they really worked, because now the menu item text was correctly turning white when the mouse was on it. Just to make sure I wasn't imagining it, I renamed the class back and sure enough the text color didn't change.
I'm not sure whose fault this is, GTK+ or Gtk#, but it's certainly one of the more interesting bugs I've seen. Who would have thought that the name you give a class could affect its behavior?