Why does an ad hoc ipa fail to install on one device, yet succeed on another with the same iOS version?

Question

I have a client whose iPad 2 (iOS 5.0) is able to install an ad hoc ipa, yet his iPhone (also iOS 5.0 -- same exact build) receives the following msg on his device when attempting to install with his Windows iTunes:

"iTunes sync. [app name] failed to install"

I've worked with this guy for a long time testing many versions of this app and he's never had this problem. All his devices install or none. How can I solve this?

Solution

As per Trott's suggestion (above), I'm moving the solution from my comment to a bona fide answer so that others can easily find the solution:

Problem solved. Device id was missing from provisioning profile. I'm leaving the issue here rather than simply deleting it bc it might help others. Must add that iTunes used to give a more specific message when this occurred, and since I had not removed the device id from the list in the iPhone Dev Center (have no idea how it vanished) I didn't consider that this cd actually be the cause. Hope this helps someone misled by the vagueness of the error msg.