For me, the worst is the Prism Framework's composition exception. There is often not enough detail to be able to tell exactly what the issue is - so you end up having to retrace through the composition of the object and its imports only to think its OK and the framework is screwing with you. Then after staring blankly at the code for an hour you finally see a missing export or an unhandled exception in a constructor somewhere and you curse Prism for not telling you that in the first place! Then you feel like an idiot for missing it for the last hour.