I agree you should always give all your knots a good tug to test them.

I probably tie over a thousand knots a month. Every once in a while, a knot just plain fails (maybe one or two out of a hundred). I think it has to do with perhaps the fact that wraps occasionally end up crossing, and the crossing wrap cuts into the underlying wrap. Not sure, I just know something happens even though I'm tying the same knots and have had plenty of practice.