Dust on the mirror turned out to be a cracked prism on a Canon FTb

I was working on a Canon FTb that came in from a guy in Portland, the viewfinder showed this weird dark crescent shape that I figured was just some stubborn dust on the mirror. Spent a good 40 minutes blowing air and using a brush, even tried a swab on the mirror surface before I noticed the crescent moved when I shifted the camera angle. That's when it hit me, the light path goes through the prism before the mirror, so what I was seeing had to be inside the prism itself. Cracked the prism housing open and sure enough, there was a hairline fracture running along the top edge, probably from a drop that the owner didn't even remember. I ended up swapping in a used prism from a parts body, cost the guy about $120 total instead of a full CLA. Has anyone else chased a viewfinder ghost for way too long before realizing it wasn't the mirror or the screen?

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