If you had been unfortunate enough to have run another round behind it, it might not have been a pretty sight.
When practice shooting, you are more apt to catch it than during competition. Both of mine occurred in IDPA matches, I didn't feel the first one, the second one I was aware of, but was pulling the trigger anyway.
I have never loaded a squib (yet), but was unfortunate to experience these from a batch of ammo loaded by a friend. I still have a bunch of that ammo, but use it only in my "time to first shot" drills (one shot only).
Glad you didn't have any damage.