App not Installed Error on Android

Question

In the Android emulator, I have software running. I occasionally create a signed.apk file and export it to my HTC Desire to test. Everything has been fine.

When I try to install my most recent exported.apk, I receive the error message "App not installed." On the emulators, everything works perfectly.

I'm not sure when this happened because I've mostly been testing on emulators and occasionally exporting to actual phones. What is the most likely reason why it wouldn't install on a real phone but would work just fine in the emulators?

Restarting the phone and deleting the current.apk did not solve the problem.