I have never had a S&W internal lock engage on me when firing. I've read of a single case of this happening But that's all.
This is another one of those urban myths that has taken on an air of credibility just because so many people keep repeating it. I don't think S&W has ever confirmed a single case of it happening. If you look at how the internal lock is engineered, it would be about as likely to happen as water running uphill. But makes for good scare tactic among folks who can't get over the politics behind the lock... I have a few revolvers new enough to have the lock, and I feel just as confident in them as any of my vintage ones. Of course the vintage wheel guns are so much sexier...