(see Updates. I've left this post roughly as it unfolded, but this wasn't an Aperture bug.)
The latest incarnation of an old Aperture bug is particularly nasty.
You get a project where only outlines of images can be seen. Click on them and you get a red square with white text saying "unsupported image format'. Restart doesn't fix this, there's no known fix. It's what you get with unsupported RAW images, but it occurs with old RAW images.
Here's one threat on the topic: Apple - Support - Discussions - Unsupported Image Format ...
There's a new release of Aperture, but no news if it has a fix. It's too big for me to download over a lousy hotel connection.
Aperture is a pro project. You can imagine how happy Aperture pro users must be to lose their work this way.
Update: The "unsupported image format" error is a red herring. I experimented with dragging the project out of Aperture, then inspecting the contents (right click, open package). It contains metadata, but no images. The images appear to have been lost by Aperture, leaving only misleading metadata. I suspect there's an image loss bug that can be triggered by moving images between projects contained in different folders.
Update 2: I'm going to run the consistency check described in Apple's Aperture troubleshooting page. I'd moved this Library from another machine, and I think that's an unsupported action. I see now I have 'read only' permissions for the Package -- I'm surprised it works at all. I have changed Package Permissions to read/write. I'm going to save a copy before I do this however.
Update 3: After I changed package permissions I ran the consistency check (Command-opt click on Aperture, hold cmd-opt until get dialog) to fix other permissions. That concluded without a message. I then restarted Aperture and this time ran the database repair. That found the images that had vanished earlier and restored the project with correct values.
So this problem began with my copying a library to a new machine, which is not entirely kosher. Then I failed to check permissions on the library/package. That meant Aperture could do some things, but other things would fail (no permissions). This particularly impacted copies on folders -- so when I tried to copy images into a project in a certain folder only some of the data was copied -- the pictures were orphaned. (This is arguably a bug, Aperture should detect the copy failure.)
The unsupported image message was a red herring.