Revealing mistake: When the builders are committing suicide the guy in charge keeps turning earlier to the areas of the falling instead of after they land. A special FX and timing error I guess. (00:06:20)
Revealing mistake: When they are sitting down for dinner at the old lady's home, it is clear that a very white light source is in the room brightening things up - something that non-electric oil/kerosene light sources cannot do.
Answer: If such an event were to actually occur in the real world, that would be the most likely scenario. The mass suicide is just how M. Night Shamylan chose to portray the event in his film.