Awkward question: should someone be allowed to continue beligerantly encouraging other people (especially beginners) to do things that are generally acknowledged to be dangerous?
We have a specifc rule about safety for that very reason. It's only been used once or twice, but it's there.
Everyone's level of "dangerous" is different. I famously say that anything over 12VDC scares the shit out of me. Then you have Electroboom and Photonic Induction.
A 12V car battery scares me in the wrong circumstances: drop metal across that and see how much disappears as vapour and blobs. Metal bracelets and necklaces are verboten EDIT: 2n3055 mentioned another classic: wrenches.
OTOH I have secondary school physics equipment where there is 230-400V (controlled by front panel potentiometer) on an exposed terminal. Touching that terminal with a finger causes the dekatron counters to spin It has a 5Mohm resistor in series, which is required so that a Geiger-Muller tube discharge generates a pulse, which is counted.
The particular problem encountered here is people telling beginners that floating a scope is OK, and that traditional scope probes are OK when connected to the mains. They never add the necessary pre-conditions, and - disgracefully IMHO - often belittle or deny them.
Traditional 'scope probes (those from legitimate sources)
are OK when connected to the mains, if a number of important precautions are taken.
In many cases, for a quick look at the mains, just to "see if it is there", there is no need to attach the ground clip to anything, as there is a sufficiently good return path to ground via the neutral/ground connection at the power inlet to the building.
Another method which was used is the "quasi-differential" one, using the CH1+CH2 setting of the 'scope.
These methods were quite adequate in the past, when differential probes were similar in cost to a small car but are probably not worthwhile when such probes are relatively cheap, especially for beginners.
One problem is that beginners get a nice new shiny DSO, but have no analog signal sources to look at, unlike oldsters like myself, who first got our hot, sweaty little hands on a 'scope in a work situation, with multiple signal sources.
We also knew that if we wrecked anything, we would incur the wrath of the Boss--not something to take lightly!!!
Our beginner has zilch, & probably hasn't read the instructions to know about the cal output square wave, they can't look at a transformer secondary (even their "wall warts" are switch mode) so their "bright, enquiring little minds" fix on the mains as a convenient source.
As to "floating a 'scope"----- in all my years in Electronics, I have seen it done once.
That was being supervised by an EE, a Supervising Tech, a Senior Tech, & most important of all a lowly Tech with his hand on the Main circuit breaker handle.
After that one test, the 'scope was immediately returned to standard configuration.
Apart from that one time, it was NO!, NO!, NO!, NO!
So if a beginner can organise the personal attendance of a similar lot of expertise on the day, it
might be OK!