2 examples.
In the first, in the 80's, I used to hang out at a wonderful dealer in Philly called Chestnut Hill audio. Jack Rubinson, the owner, and I became friends so he let me dabble and listen to equipment at will. Once , and I forget the speaker, I hooked up a speaker that had to be driven by 2 separate amps- one for the bass, the other for the mids/tweeter. Both were hooked up to the intrinsic crossover separately. Well, it turned out I reversed polarity of one of the amps and when we threw the switch on the bass amp (a Krell) there were fireworks galore. (I obviously shorted the main rails by somehow tying the positive terminals of each amp to one another through the crossover) I blew the crossover and the amp and had to pay for the repairs with profuse apologies to Jack of course. Somehow, he let me remain a valued customer and friend.
The second example was far more innocent. Some years ago, in my Dallas home, I ordered brand new ARC610 amps. I turned them on in genuine excited anticipation, eager to hear these wonderful beasts. The first one blew immediately. While that one was being shipped back to MN for repair, I decided to listen to the other amp in mono for the interim. The second amp blew after week. And that folks, was the very last time I bought an ARC piece of power amplification. Lesson learned.
Wow that brings back memories, I feel your pain. I had a similar occurrence with ARC Classic 120's. I finally sold them after they were returned from ARC for the second time with blown circuit boards. I also will never purchase another ARC amplifier. Preamp no problem, amps hell no.