Post Mortem: A single whitespace character
This very example -- requests were technically illegal all the time without devs realizing, but something in the stack changed to start rejecting them -- demonstrates the fallacy of the "be liberal in what you accept, strict in what you issue" principal.  If all the web servers involved had been strict in rejecting the illegal request from the start, they would have noticed the bug in development before deploying to firmware in the field.