BizTalk 'unknown system exception'

I've been getting "unknown system exception" on a new bit of work I've been tinkering with in BizTalk 2006 R2.

There are a number of mentions to this on forums, with a lot of links to Eliasen's blog.

This refers to a Call Orchestration shape in an unreachable branch of a Decision shape.

Another theme is using generics in a serialisable class object.

Mine though, is nothing to do with any of these, so it looks like it's just a generic error that is thrown with no especially useful information.

I was referencing a promoted context property within a message assignment shape. Although I had forgotten to put in the property name in the parenthesis. So, it was just msgInput(). I put in the property I wanted to access and hey-presto. Everything builds!