So, Mozilla plugincheck (a wonderful service, by the way) told me that "Shockwave Flash" was out of date in Opera. That's odd, as I'd just updated it in Safari, and I thought that Opera and Safari used the same plugin (I know IE is different). Anyway, I clicked the link to upgrade. The installer told me to close Opera, Safari, and Firefox before it would run (odd, as upgrading it in Safari hadn't asked me to do so). The upgrade ran, and I reopened Opera and checked. Apparently, it's still out of date.
This isn't the first time this has happened. And I've seen it in Firefox too. Always with the Flash plugin.