Good to know. Years ago I was reading an article put out by either Lyman or A-Square (which by the way, has an incredibly great section in their manual about pressures that every reloader should read) about how they do pressure testing. They do each load consisting of 10 rounds at each given charge increment in their pressure barrels. If ANY of the ten loads exceed maximum allowable pressure then that load is considered above maximum and the next lower charge that did not have any of its rounds exceed maximum pressure is re-tested.
Powders are funny animals and they do not always behave consistently and pressures can fluctuate widely, by several thousand CPU or PSI so that while some loads are a given level may be just fine, one may be enough to actually cause damage to the cartridge or rifle, flattened primers, blown primers, stretched frames…
I know we have all seen proof of this in our loadings. Everything is going along just fine then all of a sudden a round clocks significantly higher than the others and you may see a flattened primer or even a blown primer. Humans being what we are, we are very likely to dismiss this as a fluke and continue on loading, and even at the next higher charge we might not see any signs of over pressure and we delude ourselves into thinking we are just fine and dismiss the warning sign. But we are still into an area where that powder is not stable and can produce unsafe fluctuations in pressure.
That is the reason one needs to test more than one round at each level. I know, components are expensive and sometimes limited, but a broken rifle or broken body is even more expensive. Be extremely careful going up in charge with a sample size of one. You could find yourself in dangerous territory very quickly. That one in ten OP round becomes one in five becomes one in three... but if one is doing a sample size of 1, then one may be in the realm were every round is OP with extremely high, even destructive spikes, before anything shows up on the case as being out of normal.