First you must understand what you are doing, otherwise you can miss a point somewhere.
It doesn't mean you can't still miss it after decades of understanding.
One example is isolation transformer.
Maybe you have not needed it and its function has faded.
Then you get an ancient machine with a used to be a 0-class plug.
With luck you burn only a fuse.
I've measured 10A from me to ground and some real amps from RS-232 GND.
I'd say, as a rule of thumb, if you need a meter don't do it, get a guaranteed differential probe instead.
(no, I didn't)
On the other hand, connecting different cases together was a norm once.
But so was waking a battery with a crowbar and connecting bendix gear using the other method and nobody should do those eighter.
On the 3rd hand, I've learned to trust the chassis ground, but a note, do not do the same with an unknown extension cord.
And don't trust that ancient machine has a chassis even if something looks like one.
Maybe one more thing.
Nowadays buildings have 3-phase wall stuff, it means that different wall jack groups can be in different phase.
So to avoid that part of the equation use only one group of jacks or single extension cord.