Two failures in the S-76 (Turmomeca), one announced itself by a large explosion (well it seemed like it at the time) followed by smoke in the cabin - no prior indications, the other was just a soft pop - again no prior indications. In both cases bits had become detached in the hot end taking out the blades. Observed one in a S-76 (Allison), aircraft written up for being way down on trend so maintenance decided to do a power check, guess what, it let go in no uncertain manner and from the look of it very little would have been salvageable - perhaps the trend was trying to say something. Oh well, you live and learn.