The TV Vent Shaft

Why is it that every time someone crawls through a ventilation shaft on television they fall through the ceiling? I would think by now that people on TV would have figured this out. If I'm ever tempted to crawl through a ventilation shaft I'm going to say to myself, “No way, they're not designed to support a human being, although they're the ideal dimensions for someone to crawl through.” Why are they always so clean, anyway?