Oscilloscope ground lead - BANG!
Note that the 'ground' symbol on that 13V rail is NOT the same as the main safety ground symbol by the power cord. 

Look at the whole wiring diagram and notice the two triacs a little lower down, if you are designing this thing you can save a couple of opto triacs by referencing the 13V line to the incoming live....

Get paranoid, a multimeter check would have saved you some SMPSU repair work.