stsadm upgradesolution and new features

I've had a few people come up to me and ask me why they can't find their new features after they did an stsadm -o upgradesolution.

To make things clear, the scenario is as following: example.wsp is deployed and contains two features, feature-A and feature-B.

If, for example, you add a new feature (feature-C) to example.wsp, and deploy it using stsadm -o upgradesolution, then you will not find the new feature in your feature overview. Neither in the site features nor the site collection features will you find feature-C.

It looks like feature-C wasn't deployed. But wait.... the feature can be found in the 12-hyve. See what happened? The feature is deployed but not installed!

So if you do a simple stsadm -o installfeature and presto: the feature will appear in the features overview.