UDN users have reported hangs in the material editor when editing specifi material instances. This seems to be a threading bug, as CompileDebugViewModeShaders() is waiting for a shader compilation to complete which does not happen.
Attached is a sample project from a UDN user with MIs triggering this behaviour.
Link to UDN case: [Link Removed]
Observe the editor freezes. This does not always occur.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-199538 in the post.