My Nexus 4 developed an interesting one after the update to 5 and 5.0.1. Incoming calls would crash it half the time, or 5 seconds of silence on calls that didn't crash. The dialler was almost unusable.
Was widely reported but never properly resolved, so that was the cue for replacement.
My Samsung Note 3 got so laggy that I couldn't get the unresponsive phone app to actually pick up the call before it flipped to voice mail. Reboots would fix it temporarily, but it would come back. You have failed at your primary purpose, phone. Android experiment over, back to iOS.
Was widely reported but never properly resolved, so that was the cue for replacement.