My Cocoa app doesn't want to run on another computer...

I give my new app to test to a friend and it doesn't work on its Mac…
Are you sure that every library/framework your application requires, are available on everybody's Macintosh ?
It's very easy to use OpenSource libraries or third party frameworks, install them on your development machine, forgetting their presence,… and eventually think their presence is "natural"… when it is not !
Another source of problem, may be the "localization" on your friend's computer: different settings in "locale" preferences may reveal bugs in your software…
Hardcoding of file system paths - especially localized ones - is also a classic misconception of software: people may have different partitions, some of them may not be HFS ones, …
Individual
/Users folder may be on a different partition than /
Access rights may not be what you think on somebody else's computer… do your application writes info in its own bundle ?
Do you support correctly hard links, symbolic links and aliases of documents and folders ?