IR2110 - measuring Vb to Vs kills MOSFET
So, without a PE wire, the live and neutral, (via capacitors inside the scope's power supply) will "force" the scope's ground to about half the AC voltage of the supply. The scope uses capacitors because it will filter the supply for pretty normal sensible reasons.

You then apply that ground connection (now at maybe 60VAC or 120VAC) to a sensitive point on your circuit.

You shouldn't be surprised at what happened. Think about how this pans out: -



The above picture shows an "inline" AC EMI filter that uses a safety ground. Now your scope will have an earth and you, in effect, have it disconnected. Now, the two capacitors marked Cv will form an AC potential divider and the outgoing earth is at 60VAC or 120VAC. Yes it won't be enough to kill someone but it will be enough to puncture through a gate on a MOSFET.