Hmm, you're not the first one I've seen reporting that. Which version of Opera, on which platform, are you using, and how are you searching (web page, using a shortcut search, search field in URL bar, search field in personal bar, etc)?
I'd recommend you file a bug report [opera.com] about it, stating the exact circumstances where it occur.
I use Opera a fair bit and have never noticed this before (probably because I never use Yahoo), but trying it now I'm getting the same results as madcat. The example search for: "microsoft outlook troubleshooting" is turned into "microsoft outlook trouble16a" and others do a similar thing. No problem with the search for "google forum" though. I'm also using Windows 2000 and Opera 6.03.