Now and then we come across a site that does not want to collaborate with Opera. This section of the Wiki will attempt to give a few hints about how one can pinpoint the problem, where to ask for help and - if applicable - how to file a bug report.
When accessing a web site you are taken to a page saying "upgrade your browser", "please download Internet Explorer to use this site" or similar.
See Opera's Open The Web project
This is a slightly "kinder" form of the above - the website gives you access but detects Opera and thinks "oh, this browser probably can't show my fancy menu so I will just disable it". This is a very common reason why things do not work and is often related to commonly used scripts not being updated.
See IdentifyingInternetExplorerAsOpera
Because Opera identifies as IE by default to get around the two problems above as much as possible, sometimes a website will try to use IE-only features such as plugin detection with VBScript or "CSS filters" triggered from a JavaScript.
Oh yes, if something doesn't work it may also be a bug in Opera...
For the last two categories (and actually the previous ones as well), see the JavaScript debug wizard.
If something doesn't work, these are the first things to try..
If you have asked others and many people seem to believe it may be a bug in Opera, do file a bug report. Do read the explanations carefully if you are not used to bug tracking systems. Try to write a concise description of what happens and what you expected to happen, and include any error messages in the javascript console. http://www.opera.com/support/bugs/