Well, I just had a fantastic time tracking down a weird problem that was stopping one of our applications loading. Right before a demonstration, one of our machines started giving an error message "Cannot convert string [png file resource path] in attribute 'LargeImageSource' to type 'System.Windows.Media.ImageSource'. Operation caused an invalid state. Error at object [item name] in markup file [xaml file]".
After a bit of chasing around, I noticed lots of people lately having issues with KB2670838 update. Apparently this update broke some of the image loading functionality in WPF, as well as causing blue screens, turning roads in Battlefield 3 yellow and causing the UDK editor to fail to load. So on the problem machine, we uninstalled the update and everything went back to working normally.
What's really odd is that I have this update installed on my system (both the systems in question run Windows 7 x64 SP1) and the application continues to function without problems. But that I can tell, I'm at the mercy of Windows updates randomly breaking my WPF applications without notice or obvious recourse.