This evening I noticed my P3S had a horizon way off kilter, maybe by 4-5 degrees. I brought it down to a hover about 5 meters above ground level and noticed the craft itself was tilted--this wasn't a gimbal problem. Also, it was having trouble holding its position during the hover. I expect some roughness down low due to prop wash, but I'd call what I saw downright erratic. I landed and was about to do an IMU calibration when I noticed one of my props was sitting a bit higher than the others. I checked to see if it was tight and I pulled it straight off the nut. Oops. That prop will not seat securely on the motor at all. I must have damaged the threads last time I took it off or put it back on. (Yes, before you ask, it is the right prop.) I count myself fortunate that my drone didn't fall from the sky today. I drew a big red X on the damaged prop, replaced it with one of my spares, and was back in business with a quad that was flying level again. Lessons learned: Always always check that the props are correctly installed and tightened before every flight. Be deliberate and cautious when installing and uninstalling props. I'm not sure how I damaged mine, but I suspect I may have cross-threaded it at some point. If your drone isn't flying right, bring it back home and investigate! At least I had the presence of mind to do this today, even if I failed at the other two.