You can always get older versions...you can even have several installed at the same time and compare results.

http://www.opera.com/download/

But... I have had no problems, and if there are reports of problem, Opera has a history of fixing those which are real, (and not just a result of incompatibility with sites that specifically ignore standards in order to cater to IE or Firefox).

Read http://labs.opera.com/webstandards/