I was responsible for a driver for a router ASIC. Had some reports of odd behaviour coming in from the field so I setup a test to reproduce it. Sure enough an interrupt I hadn't seen before goes off deep in the ASIC. This was repeatable, with the right traffic pattern it would fire off immediately.
I opened up the manual for the ASIC and after a while of searching finally located the section describing the interrupt. The "Cause" and "Recovery" sections were blank, and the "Description" contained one line
This will never happen.