I did that once
And my daughter-in-law was shooting my son's new Smith. Like you, we were all lucky that the slug only lodged in the forcing cone and prevented the cylinder from rotating. None of us knew what happened, only that the cylinder was locked up. Took it to the local gunsmith. He chewed my butt. I NEVER seat the bullets without visualy checking every case for powder. Only takes a few seconds to look at a tray of brass. If the bullet would have lodged in the barrel, that would have really sucked. The young lady didn't understand the squib load, and pulled the trigger after that round.