Bugs are usually annoying, but sometimes they can make me laugh.
Take this image for example. I must have right-clicked just as Mozilla Firefox loaded and became visible, although I had intended to click on whatever window had been there previously. The context menu that came up was fucking crazy.
Why? Well, maybe because the program lacked the context to know which list items to show. This suggests to me that the context menu is populated with all possible items (including those from addons) when the program loads, but they are hidden and shown as required depending on the context of the right click.
Whatever the cause, it was a nice WTF moment to my day ((FYI, this happened on 22nd February 2010 at 22:06.)).