Broken lens cover. Must be at least 50 characters here.
Easier than the video. However, the cover had protective on both sides, and I thought one side had an adhesive, so I had some tough moments flipping it over without touching before deciding there was no adhesive. the cover slipped down inside without any effort (unlike the video), and putting the metal cover on was a snap - actually no snapping but no real effort.
Other than needing to know to pry up the bezel, and to just crush the glass in order to scoop out the goo, a written paragraph would have done a better job than the video.