If I'm doing adjustments or setting up connections for troubleshooting, I'll just unplug so it's a no-brainer. If I'm actively probing around while it's on, I just be mindful of my body and hands/tools at all times. It's very easy when your head's upside-down and backwards inside these games to accidentally bump into something or drop a tool or do something else stupid.

Be mindful too that depending on the game, the direct line voltage from the wall could be snaking throughout the game (and not necessarily all localized to one area like near the transformer).

It's a good point on where to focus your time. When I'm troubleshooting the average problem, atleast 50% of the time is spent tracing the schematics and figuring out where to put the probes. Power-up the game, collect the data, then go back to the schematics. Of course there are exceptions ..

Got any examples of when this would be useful? Just curious .. I breadboard a lot for other (mostly digital) projects, but haven't run across any problem yet on an EM where it would make sense to break out the breadboard.