I was testing compression with a friend and the first cylinder went smoothly.  However, when my buddy and I did the second side, we forgot to undo the spark plug wire to the opposite side that we weren't testing.  We cranked it over a few times then, Bang  !  It fired and was as loud as a gunshot.  It also made a terrible burning rubber smell.  His future father-in-law came out and asked us to quiet down.  No harm done though  .  I will however always remember to disconnect that wire in the future.

Its better to remove both plugs anyway. Makes it easier for the starter to turn it over.

also open the throttle all the way when doing compression tests.   if you have the equipment though, a leakdown test is much more useful

Chaulk it up to another lesson learned with a surprise!