In the post A Month of Nexus One, Heikki Toivonen lists some the bugs he has encountered on the Nexus One : "I’ve encountered some annoying bugs as well. During my first week the phone got into a state where tapping the messaging icon always launched the browser. The only way I could recover was to reboot. This hasn’t happened since. Far more common problem is the soft keyboard not registering taps, or thinking I pressed one of the four buttons at the bottom (back, menu, home or search, with accompanied vibration feedback). Switching back and forth between apps fixes this eventually. I also seem to have situations when I don’t seem to get any GPS information. Some people have reported that this might require a reboot, but I haven’t been trying persistently enough to confirm if this is the only cure.".
I also encountered some problems with the four soft key at the bottom, where they don't seem to work at all, or when they seem to trigger the same action, or even when touching the screen in the middle triggers one of the four soft key button ... In those situations, the best thing has been to reboot the Nexus. It seems this happens more often when the phone has been up for some time, or when there has been some weakness in the network (I use the Nexus during my commuting time and there are a lot of places where there is no or poor quality network that make the Nexus to frequently switch between Edge and 3G).