450v on 400v probes in to a 300Vrms Scope 
The probes on my DS2072 are rated at 400V. Was measuring a 450V pulse with a 1uS rise time and blew it up. Note to self: probe voltage rating is also frequency dependent. After that I got a pair of dirt cheap ebay x1/x10 probes and also a pair of 2kV x100 probes. On the rare occasion I need real bandwidth I use the good probe. Day to day hack stuff the cheap probes get used.
Well,if he was using a x10 probe,all the 'scope will see is 45volts.
Besides,300v rms is around 424v peak,so he almost made it,as far as the 'scope was concerned,even with a x1 probe.
All the old Tek equipment was very conservatively rated,so you could over step the mark a bit---it was usually rated a bit higher,anyway, than modern stuff.
x100 probes were unheard of in the old tube days,when "400-odd" volts wasn't at all unusual