High voltage seen when scoping with diff probe

[Faringdon]

Hi,
Today i was probing a circuit where all voltages should have been less than 15V. But the diff probe showed up over 100V where the arrow points in this picture...and LTspice sim.
It was as if the diff probe's internal 4MEG was connected to mains line, instead of to earth.
What else could have caused this high voltage to be seen?

I was probing from Collector to emitter of the BJT,  and saw the 100V there, which surely couldnt have been there but i saw it.

[Gyro]

With a 10uF capacitive divider, where is all the current going to cause the rail voltage to be a mere 15V?  I could make a guess at shorted 40V transistors, 555, opto LED etc.


EDIT: This may be related to  https://www.eevblog.com/forum/testgear/scope-waveforms-out-of-sync/  who knows.