This is actually a bug report for Silverlight on Mac OS X, but the required OS field didn't offer Mac as a choice.
We develop a web browser application for Mac using the built-in WebKit framework. We've discovered that when a page containing Silverlight content in loaded in a WebView, Silverlight crashes. We've been testing with the latest Silverlight for Mac version 5.0.61118.0, and we've been able to reproduce the crash with just a sample Xcode Cocoa application project that links to WebKit and has a WebView in a nib. On the i386 architecture, loading Silverlight makes the app itself crash. On x86_64, it makes WebKitPluginHost crash. I'm attaching some crash logs.
It seems that other developers have run into this issue as well:
The crash occurs on both Lion and Snow Leopard if the project is built with Xcode 4. The crash also occurs on Snow Leopard if the project is built with Xcode 3. Strangely, the crash doesn't occur on Lion if the project is built with Xcode 3.