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.
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)
Answer: It appears so. M. Night often uses the color red in his movies as a visual clue that something emotional or violent is about to take place. If there was blood on the swing, it could be a forewarning to the audience that the house in question is more dangerous to the main characters than originally assessed.