Comment 8 for bug 1508096

OK! This does indeed affect master. I've found the following step to reproduce it:

1) Create a simple stack with a OS::Heat::TestResource
2) Update the stack, adding a MyResource referring to some resource in the environment, and updating the TestResource to fail: True
3) Update again the stack without the fail.

I think I've also found the fix, as it sounds fairly similir to bug 1477812.