The site patching blog had a new update last week about a type of problem that seems all too common:
For some time we have seen compatibility issues because Opera supports both addEventListener and attachEvent. In some cases this causes scripts to be run twice, in other cases attachEvent is simply used as an IE-detector, causing Opera to run the wrong codepath or get IE-only stylesheets or other bad things.
So what are you going to do when you are, in a way, being "too compatible"?
Well, with Browser JS we can test possible changes to Opera's core while still being able to quickly remove the changes if they turn out to worsen things.
But once again, this shows how incredibly complex site compatibility is. There's no silver bullet. Even trying to be "too compatible" can cause problems!