I was in the middle of playing some Ingress out on Folly Beach today when my Nexus 5 suddenly started acting crazy with random phantom touches on the screen. Things were moving, menus were activating and all kinds of weird things going on without me touching the screen at all. Very strange, making me wonder what the heck was going on. I wiped off the screen to make sure it wasn't drops of water or sweat messing things up, but no luck. The phone was pretty much useless and I didn't want to stand around outside in the hot trying to troubleshoot, so I headed back home.
After getting back home and going through some routine troubleshooting steps (power down, cold boot, a couple of hard resets, all of which aren't that easy to accomplish when your phone is going nuts and not responding to screen touches), I decided that perhaps some kind of hardware corruption had occurred.
I put in a call to the Google Support line and after waiting on hold for a few minutes, was connected to a very pleasant sounding lady. I described the problem and my own troubleshooting steps, and one of the things she asked was if I had a screen protector on the phone. I told her there was one, but it had been working just fine with the screen protector on earlier and didn't think that would be the cause of the problem. Since her troubleshooting steps were pretty much the same as what I had already done, she put me on hold to process a replacement order and exchange.
While on hold, I started messing with the screen protector to get some small annoying dust bits out, and figured I might as well take off the screen protector just to see what would happen. I wasn't expecting that taking it off would make any difference at all. Much to my surprise, the phone started working again and responding to my screen touches.
Upon closer inspection of the screen protector, it looked like a tiny bit of moisture (probably from my sweaty fingers on the phone while playing Ingress) had wicked into one of the small dust bubbles near the edge of the screen. That was apparently enough to cause the phone to read all kinds of random screen touch activity. Once that little bit of trapped moisture was gone, everything was just fine.
So with that, I think I learned a bit of a troubleshooting lesson, which I should have already known: never discount even the seemingly trivial possibilities.
Now for the laborious process of restoring the phone.