Are you using Lee equipment?
I've had this happen to me when using my Lee Auto Disk measure. I like it, but if you're not belling the case enough, then it will sometimes throw a light charge. That's easy to check, though. I've never had a squib because of this adjustment, just something I know to watch when I setup my dies.
I've also had the same measure throw a light charge when equipped with the adjustable charge bar set to LESS than .4 cc using a flake powder. At .4cc and over it's great, less than that and mine will throw a squib or two in .38 special. Lee warns against using flake powders in the charge bar when set to less the .4cc, I thought Bullseye would be ok, but I was incorrect. Stick to the cavities when loading under .4cc and for really light loads use the micro disk.
Other than learning my measure, I once checked the powder and then forgot to load the powder.
Finally, I had squips in .357 magnum once with Russian primers that absolutely refused to ignited Blue Dot. I ended up with partially burned gun powder down the barrel and a bullet lodged in the bore. I switched primer brands and all is well.
Last edited by testuser; January 2, 2013 at 10:59 PM.