The classic bed of nails demonstration (with cinder block burst) can sometimes end badly. My friend and Skeptical Teacher, Matt, does the bed of nails often enough to experience bad consequences from time to time.
Here's one that went bad two ways. Can you identify the two unfortunate events and which one was more regrettable? (The "good stuff" hits at 5:20ish.)
I hasten to add that neither bad result was really Matt's fault. But I'm guessing each one represented a learning experience for him. And teachers appreciate nothing so much as learning.