If you try to include a period (.) when you name an album, book, web journal, or web gallery, Aperture will display this message:
Rename Error
That name is already in use. Please choose a different name.
After you click OK to dismiss the message, the period and anything following it will be omitted from the name. For example, "My.Book" would become just "My" in the name field.
Although the message is technically erroneous, the request is valid: To prevent potential naming issues, you should avoid using names that contain special characters or punctuation marks, such as periods.
For more information, see pages 49-50 of the Aperture Getting Started guide.
Learn more about naming files in a cross-platform environment.