From a short lunch-and-learn session at ConstructConnect: A discussion on why it's important for testers to understand trust boundaries in the systems we work with.
Input processing is complex and expensive. We can't (and shouldn't) do that in every single part of a system, so it's critical to work with developers/architects to create and understand system boundaries. This makes our testing much more effective as we know where to concentrate our efforts.
This L&L session was interactive, so I posed a bunch of questions to get responses, then guided some discussion around what trust boundaries might look like.