This took a decent amount of debugging and headscratching to figure out why a particular .NET call kept failing.
I inadvertently discovered and wired a "vestigial" output terminal that exists on .NET Constructor Nodes in LabVIEW 2012. Interestingly, this output terminal returns the same datatype as the "real" output terminal, yet does not return a valid ref.
Pretty sure this is a CAR-able bug, but just documenting behavior here just in case someone else runs into this. (Can you think of possibilities to exploit this as a feature??)
Here's a 47sec YouTube video with no sound; Vestigial Terminal at 25sec:
...and for the sake of organic search: Error 1172 - "LabVIEW: A .NET exception occurred in an external assembly. For information about correcting this error, copy the following exception (in bold), and search the Microsoft Developer Network (MSDN) Web site or the Web for a possible explanation."
And in case the resolution for this "error" and my "bug" was not clear: I just had to rewire the terminal to the "correct" output. Further, my choice of .NET assembly, constructor and property for the demonstration was arbitrary; this appears to affect all .NET Constructor Nodes.