Things I don't understand. Mains and tube amp get me thinking twice about every move I make, but if you know what you are dealing with, you also know how to do it safely. But if there is something on the bench and I don't know if it is isolated, or how it is wired up, or what voltages are present, that scares me. Where does the mains go? Are these huge-ass DC caps on the primary side of the DC/DC discharged on power-off?
That and Airsoft/RC batteries. These things don't come with current limit and the good big ones will happily deliver 250+ A on short... I've had to work on airsoft guns a few times and I always make sure to check for shorts 20 times, as well as powering it up with a lab supply (but not firing, since those guns can draw near 100 A peak, 40 A continuous) to check that there are no actives doing strange things.