In my iLemon post about my iPhone 4, one of the particularly annoying bugs I noted was the custom ringtone bug.
Sometimes a custom ringtone will continue playing in a stuttered loop even after I answer the call. A garbled few seconds of the ringtone keep playing, over top of the other party saying "hello, hello?" The only solution is to hang up on them and call back. (I've also had trouble playing the Apple ringtones in Settings > Sound > Ringtone > Standard, they sometimes stutter and skip too).
Here's a video of the i4 custom ringtone bug in action:
It only happens once in a while, but when it does, my i4 will ring and a snippit from a custom ringtone will continue to play in a repetitive loop. Forever. The fix is to either reboot the phone or to wake it from sleep.
One fix is to reboot the iPhone 4 every 24 hours. It's a ridiculous requirement (what is this? Windows 2000 server?) but it definitely helps with iOS4 on the iPhone 4.