A Black Widow.
My friend liked to keep things clean in her house, but as a single parent with six kids to look after, she sometimes couldn't get around to everything and everyone as often as she would have liked. Her bathroom, in particular, had poor lighting so it had taken her a while to notice how dusty it had become in places.
When she went to straighten it up, she discovered an unwelcome inhabitant living on the underside of her toilet tank: a female black widow spider. She immediately killed it. There was also an egg-sac nearby that she destroyed. She then went over the rest of the bathroom in a systematic fashion, discovering three more egg-sacs identical to the first in a disused corner of the closet.
The egg-sacs weren't dusty, though. They were all empty.