This was my first thought (and first experience in 60 years of reloading) as after firing my first ever shot in my new to me 7x65R Heeren (and thinking that it was a potent reaction to expect from 10 grains of Unique behind a 150 grain Loverin cast bullet) I opened the action quite normally and spied the shiny case head and mirror flattened primer smoked around it and the extraction stopped before it even got started. Wow! I tapped the brass out with a rod and at that point the primer just fell out. I noted that the headstamp on the new Norma case was oddly faint compared to when it had entered the chamber. My first assumption was that with so little powder in the long and fairly large case that I had failed to settle the powder against the primer as I had chambered the round and that it had been strung out along the length of the case and the primer had somehow done a full immediate ignition or something. I trusted my loading routine as I had only the Unique powder out and I had, as always, done a flashlight inspection of the powder level in the 15 loaded cases before I started seating bullets. The load was 10 grains of Unique and I had checked my recorded Harrells setting by dumping 3 loads onto my scale giving a reading of 30 grains which was then dumped back into the top of the Harrells bottle before I started dumping straight from the Harrells into the cases in the loading block. All the bullets were seated for the cases I had just put the powder into and the loaded rounds placed in the ammo box before I emptied the powder out of the Harrells bottle back into the Unique bottle. The Harrels bottle and the Harrells itself were definitely empty before I loaded the next powder I was trying, SR 4756. So, I wasn't worried as I carefully made sure the powder was at the back before I loaded the next round. That fired as would be expected and extracted (actually more like ejected, the extractor really works well) normally. This continued for a few more shots and then I realized that my jury rigged scope mounting had loosened so before I closed the action on the case I had chambered I tightened up the scope mounts. This time as it fired I realized that I had a repeat. Sure enough a little smoke around the primer and case had to be rodded out. Not like the first one though. So had the time spent working on the scope mounts laid the powder out and invitied another excursion or? That was enough for the day. To be continued.

Question: were any of the cases found to be loaded under the 10 grain amount?

After years of reading of unexplained incidents similar to yours my steadfast loading method is to drop the powder into the scale’s pan and once the weight is verified I then funnel the powder in the case, a visually identified empty case, and seat the bullet immediately thereafter before attempting another load. Have yet to overcharge a reduced load or for that matter undercharge a load.

Amen to experience but there comes a time were one is not as attentive as one used to be and for me that is the scary part. I catch myself all the time wondering if I charged the round correctly and pulling the round down to look for an error. So far no failures but the day is coming for those failures and that’s the part of life I’m not looking forward to, Lord keep us safe!

Only time I ever got an overcharged case was when I loaded cases the day before a match. I know of another shooter who wrecked her rifle with pre-charged cases. 
Be careful!

My routine is to always use at least half a hopper's worth of powder even if I'm only going to load 10 rounds. After putting the powder in I give the hopper a minute of rat-a-ma-tazz slapping with my palms front to back and side to side until the powder is well settled. Then I dump 15 loads in a funnel and dump that back in the hopper (makes very little difference going into a half full hopper.) Then I dump 3 loads in a scale pan and check the weight. That goes back in the hopper and I'm ready to start loading.

I keep a funnel in the powder jug. Anything not going in a case goes back there. The hopper on the measure stays sealed.

I generally check weigh most charges. And I do catch bridging in the drop tube now and then. The measure gets 'knocked' to dump it, and cycled a couple more times before going back to the pan.

Almost all modern stuff, 223 with N-140 the largest volume. Hundreds of rds at a time. The short range stuff gets eyeballed at the case mouth as good enough. Same routine if it's not all there, just no scale in the loop. Other than a check every 15-20 rds.