Today, I know what the problem is: some strange bug in the Microsoft backend sends a load of unexpected NULL characters – more than two million of them right in the middle of the source code. This sounds (and is) bizarre for a high-profile production service, so here's a screenshot to prove it:
In front, source code Opera gets, viewed in SciTE. Behind, a comparison of source sent to Opera (on the left) and Firefox (on the right). The comparison tool ExamDiff is loaded directly from the Fiddler HTTP debugger to prove that this is about bytes coming from the site rather than the result of some strange browser-internal bug. (Line numbers in ExamDiff are greater because it also compares the HTTP headers).
Wow. So you're sending more than two million nonsense characters to every user? Somebody working late hours at Microsoft recently?
Opera gets way more confused by the unexpected NULL characters than Firefox does, judged from the CPU usage. We should fix that – meanwhile, here's another strange but great example of how testing in multiple browsers can make you uncover mistakes you never expected to make.