Had one similar whoops a number of years ago, one reason I've been very careful about checking the scale ever since! Have also seen some errors in electronic scales, so it isn't unique to balances.
Oddly enough the whoops I had didn't show ANY signs of high pressure for the first shot--other than a very high chronograph reading. The chronograph I was using sometimes gave screwy readings in bright light, but the second shot blew a primer, and only then did I suspect something might be wrong. The rifle was a Ruger No. 1, which proved very resistant to gas blow-back--but also opened with relatively little effort, contrary to what we often hear about No. 1's.
I replaced that chronograph soon afterward, but have seen plenty of problems with those belonging to others. Also oddly, for a long while it was probably the best-selling chronograph on the market, and may still be, since it's once of the least expensive. Many people have had fine results from them--including me, until they went screwy. Yes, "they," because I owned three before finally giving up.
