Sam Goldwasser wrote:
What is the device being tested? A TV? If so (as well as other equipment),
it may have a HOT chassis, which is extremely dangerous to work on without
an isolation transformer. In addition to being electrocuted, there may be
fireworks when attaching the scope ground to signal ground.

That's how I blew up the very first TV I worked on. It was a learning
experience. It cost me a $250 20" Stereo Sharp TV (long while ago) and
was an amazing display of colors and smells along with sparks, flames,
smoke, etc.
Those days were fun.