I've loaded and fired 10's of thousands of my own reloads.
With rifle cartridges I hand weigh every charge but with pistol cartridges I've been weighing every tenth charge (after I have the thrower set and working consistently). If when I weigh a load and the charge is not exactly what I expect I go back and redo the proceeding 9. Remember many people do these on a progressive press.....
It seems highly unlikely but I must have made a squib load and thankfully the pistol was strong enough and the loads light enough not to blow up with the next rounds.
OK then David.
For 357 mag and 158 grain LSWC, can I suggest Unique. It is accurate, consistent and meters well.
I don't remember off hand, but a starting load would be 3.3 grains of Unique, and you could make the bullet go up to like 900fps with this powder. You'd have to work up your own load. Check your manuals.
Universal Clays does have a wide spectrum of calibers it is great with, but not specifically 357 mag. Especially not with lead bullets.
38 special, OK... 44MAG, OK... 357MAG, not so good.