Here is a good one - if you forget to update the version number in your iPhone app (through the info.plist file): and then try to install the App to your iPod/iPhone as an Ad Hod distribution, iTunes will prompt you to upgrade your app; but will actually fail to upgrade the app at all. :)
In this case, you could either first delete the app and re-install (but this would lose any app-specific data of course); or re-build with a new version number!
On another note, if you try to just zip-up your app with the Compress feature in Finder, and send that - it won't capture enough information, and will fail to install on your ad-hoc target devices. Instead, you should copy your app file to a folder, use Disk Utility to create a DMG file from that folder, and then zip-up the DMG file; and send that on to your testers. That will work. :)