title: application not installed error on android
body: <p>I have been downloading different apk files and putting them on my android device with no problem.  however, there is one apk that has the error "application not installed."  i have tried downloading different versions of this from the internet (even using ones that other friends have used and worked for them).  i can't seem to figure out why this ONE apk file will download on my phone but not install.  i don't quite understand all this "signed" and "unsigned" business, but i figured if my friends have used the same exact apk it should work.  any ideas why this one app is causing me such a headache?</p>