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?