Part of establishing an electrically safe work condition is to test for absence of voltage.

Have you ever thought a circuit was "dead" only to find it was not?

Yes - Found out during testing
Yes - Found out the hard way
No

Yes! On a couple of occasions. That is why verifying absence of voltage is such an important step in establishing an electrically safe work condition.

One case involved a 480V switch that failed to open properly even though the handle felt like it opened. We were quite surprised when the voltage was still present. The second time, there was a "mystery" circuit that was unexpected and not on any drawing. A very old installation.

I would not want to find this out the "hard way" as the question asks.

Twice in fact.
THe first time was at the start of my time with the first job I had out of college. We tested a circuit dead (Using a Magneblast ground and test device) Racked the G&T device out applied the grounds and racked it back in, "boom". In the time between testing, and rackingthe G&T back in, some one re-energized the MV circuit.

The second was a circuit I was told was "dead", 100%, for sure dead. 4160V breaker cubilce with the breaker withdrawn. [Again a Magneblast]
My non-contact voltage detector said different. That rated a "oh sorry, yeah I guess it isn't dead."

Those who found out the hard wa,y are not as likely to be ABLE to respond.

Even when we test, we got to be very careful especially when we have MV & LV system in the same facility. In Jan 2001, in N Carolina, there was a sad story that a gentleman who accidentally hooked multimeter on 2300V starter disconnect, and thought that he was on 600V side. He used multimeter to check if the fuse was blown out. The 1kV rated multimeter have no change against 2300V and it vaporized causing tremendous arc flash. It cost his life. We all should learn from this and should not let it happen to anyone. Since, the article was posted on youtube, m sharing the link here. May God bless his family and may his soul rest in peace:
https://www.youtube.com/watch?v=hfnEuRA7-vo

Almost. He was never trained that there was any difference between 2300 and 480 V, so he used his meter as he had always done in the past to check for voltage. He didn't think he was in the low voltage compartment at all.