Cannot start Unity: "Project incompatible"

I tried to open a project in Unity 3.5 that was made with a newer version. So I upgraded to… well, the last version of 3.x available.

Now, when I try to start Unity.app, it gives me that error about the project being incompatible. If I hit continue, it crashes. If I hit quit, it quits. How do I get Unity to start again?

Info: I’m using the Mac version.

Hold down alt/option when launching Unity. Afterwards, turn on “always show project wizard” in the preferences.