Ha! I learned the same (hard) way. As a teen, I had a part time job fixing computer terminals and printers (this would have been around 1980). I was pretty comfortable using a scope, until I had to troubleshoot a bad power supply on a Silent 700. Turns out this was my first experience with a switching power supply, and I didn't understand that the high-voltage section wasn't isolated from the mains. Tried to connect the ground lead to what seemed like a reasonable place on the power supply and BAM - scared the crap out of me, rendered the PSU fubar, and taught me a lesson I'd never forget!@Yonki, if it is any consolation, I understood right away what you meant when you said you needed to get an isolation transformer. I faced the same problem when diagnosing a Sony TV from 1980s where the internal ground was connected to the hot side of AC power! For anyone unfamiliar with using an oscilloscope, if I had attempted to connect the scope's ground clip to a ground point in the TV circuit in the usual manner without plugging the TV or the oscilloscope into an isolation transformer it would have immediately shorted out the 120V power at the full current capacity of the circuit, causing a big spark and probably burning up part of the TV circuit or the 'scope or probe.
Thanks for sharing!