I just had my first crash a couple of hours ago...
I was flying in my office's warehouse bay that has 30' metallic ceilings. Although roomy, there's large refrigerators, compressors and machinery in proximity.
I didn't get a GPS signal before taking off and I ignored warnings for high electromagnetic interference.
After two minutes of flying the mavic in and out the door of the bay, I got another warning for high electromagnetic interference. I flew the mavic towards me to land it, but at the last second, it beeped, I lost control of it and it flew sideways into a bunch of cardboard boxes. I panicked and went to retrieve it without turning the motors off and hurt my finger.
Fortunately I only shed a bit of blood and only the propellers got damaged, but this was a huge learning experience. This was my first time flying indoors and I'll be EXTREMELY cautious when attempting to do so again.
After, this I saw DJI sells propeller guards for $15, at this point making such investment sounds like a no-brainer.