Greetings Ghost Fans,
When the Haunted Mansion first opened at Disneyland, there were a few small enhancement props not there today.
One small enhancement to the attic were randomly placed black strings. In the shows low lighting conditions, the black strings were virtually invisible. As the doombuggies crept through the attic, the black strings would hit the guests giving the feeling of spiderwebs brushing against the skin.
According to X. Atencio, it was an impressive effect, but the more that guests would go through, they would soon figure out where the strings were and within hours, all of the strings would be gone.
NOTE: Thanks to the posters at MiceChat for giving a great idea for a topic!