When reinstalling the original software on an older Core 2 Duo MacBook Air the other day, installing the OS itself (in this case, Mac OS X 10.6, which came on a tiny USB stick) went without a hitch. The USB stick's iLife installer, however, kept failing with an unknown error and told me to contact the software manufacturer.
Fortunately, that wasn't necessary.
A look at the installer log (hint within a hint: when in trouble, always check the log) revealed that the culprit was an expired installer certificate. When a certificate expires, OS X doesn't trust it anymore, and thus refuses to install.
All I had to do to work around that error was change the system date back to shortly after the MacBook Air shipped. After traveling back in time, Mac OS X trusted the iLife installer certificate, because it hadn't expired yet back then. So the installation went without a hitch, and after it was done, I changed the date back to the present.
(Note: Newer MacBook Air models let you restore iLife from the Mac App Store. This model, however, predates the Mac App Store, so I don't know whether that would have worked. And I didn't want to use my Apple ID for this anyway, as I'm about to give away the MacBook Air.)
Mac OS X Hints