Was it making any noticeable noises before the test Tim? Unless the guy walked down the length of the system thumping the joints/welds with a mallet it's possible a weak joint simply chose today to crack open but looked as rusty as the rest of it with no obvious signs on test day. Rust on a mild steel car/van exhaust looks horrible if it's just surface corrosion or worse after all, and as it's difficult to tell how thin the metal is unless you give it some with a persuader or screwdriver.
At least you can fix it yourself cheaply which is more than most of us can manage
.