About four years ago I had a similar incident with a scale of similar construction but different manufacture. I was working up loads and one of them took a little extra trickling to come up to weight. Instead of dumping it back into the measure and throwing another charge I poured it into the waiting case. I do not know how far over it was, but I wrecked the end of the bolt. The bolt opened hard, but not pound it open hard. I did not have the Chrony set up, which is extremely unusual for me. It was the third round fired and was only 1/2 inch out of the group and high.
The cause was a little dust in the pivot. The scale had been left uncovered in a dusty area. Well, actually, the cause was compound operator error. I left the scale uncovered in an area with enough dust and I ignored the need for more trickling than usual. Your Herters scale is actually a Redding OEM and one of the better (IMO) available balance beam scales. Take a soft tooth brush and clean the serrations on top of the beam with alcohol and clean the pivots (beam and body) and it will be good to go. Keep known bullets (for low weights use #4 buck) with the scale as check weights. I use #4 shot, #2 shot, #4buck, 32 grain hornet bullets, 45 grain bullets, 55 grain bullets and 80 grain bullets to check what the scale tells me about powder. To check the scale at bullets weights I just compare to bullets out of a different box. Every now and then you get a bullet(s) that makes you go Hmmm. Probably comes from people opening a box in the store and putting a bullet into the wrong box.
Accidents like these are why you develop habits when you load and you pay attention when something breaks the pattern/rhythm. I have loaded virtually all of my own ammo since 1956. I was taught to set the measure up for the charge, throw the charge bump the handle on the knocker twice as I drop the charge into the scale pan, and trickle up to weight on the scale. Pour the charge into the case, seat the bullet and repeat. To this day I get nervous throwing charges directly into the case even though I have tested my measures many times and I know it is safe. I have never charged a loading block full of cases and then moved on to seating bullets. I'd probably have the twitches for days if I ever did that.
An absolutely consistent pattern loading would have prevented your whoops and mine. It will serve you well because when you get the feeling something was different your only option should be to pull the bullet and start over. Back when I started loading, things were more crude, loading manuals were not infrequently off in what was safe, how fast t was going, and you had almost no way short of firing the load to check up on it. Nobody had a chrony. Like me, you made two mistakes and didn't catch either one.
