28 years ago, when I was in Tech school, we were given components (transformer, 4 diodes, capacitors, etc.) to build a rectifier. For safety, we had a Variac to "slowly turn up the AC" so you could watch and record the waveforms from a scope. As we cranked up the Variac, the waveform didn't look right. So cranked it up further and POW! One of diodes split in two.
Instructor comes running over. Looks at the circuit and sees nothing wrong with the polarity, wiring, etc. Put in a new diode and this time the instructor cranks up the Variac and POW! The new diode splits in two. Other students come over. This is exciting. Instructor rubbing his chin, his brain spinning. After about a minute he figured it out.
The scope ground lead was not connected to circuit ground but to the cathode of one of the diodes. However, the scope ground lead was forced to ground through the 3rd (ground) prong of the AC plug through the scope chassis.
Just then another POW! was heard across the room. Instructor says to me, "Go tell him what he's doing wrong". Then another POW! The lab experiment was stopped and the instructor gave us a lecture about Test Equipment and Grounding. Having the lecture before the lab might have saved a few diodes.
creakndale