Aperture 3 - Full screen mode "stuck"

Apologies if this subject is being discussed elsewhere; all I've found are a few orphaned threads with no answers.

The problem: when I toggle into full-screen mode (for, say a single image) I cannot exit. The only way to get out of it is to quit Aperture.

This has persisted for a few weeks now and is making the program almost unusable. Any thoughts/advice?

Thank you,
David