We spent half an eternity this week trying to debug why on Earth Opera 9 doesn't allow HTTP uploads. After thousands of trials and errors, we found that if the Opera 9 is the "strict" mode, it doesn't recognize form encoding set to enctype="MULTIPART/FORM-DATA" and therefor doesn't allow HTTP uploads to complete. The nag? As per strict HTML specs, the "MULTIPART/FORM-DATA" has to be lower case nowadays, meaning that in its default mode Opera 9 wont allow HTTP uploads with forms created using old HTML specs, where the "MULTIPART/FORM-DATA" is written in upper case.
Case closed, our fault, but one of those things that you don't exactly expect to bump into nowadays :-)
I, also, have found that Opera 9 pisses up pages made with any minor bug in the code, while all other major browsers show the page correctly... And there ended my Opera 9 support.