During the Doctor Who Season 5 story arc, the cracks in the universe absorb a number of creatures, erasing them completely from time:
- Amy Pond's mother and father.
- The military clerics Phillip, Crispin, Pedro, and Marco in "Flesh and Stone".
- The Weeping Angels in "Flesh and Stone".
- Rory's corpse in "Cold Blood".
- Also, apparently, the ducks in the duck pond. ;)
However, there are 5 characters/species that are able to be close to the cracks, in some cases for long periods of time, without getting erased from time:
- Amy Pond, who had a crack in her room for years but never got absorbed by it, even though her parents did.
- The Doctor, who reached into a crack to pull out a piece of the exploding TARDIS in "Cold Blood".
- Prisoner Zero, who was able to escape prison by fleeing into Amy's house through the crack in her wall.
- An Atraxi, who was peering at the Doctor and Amy through the crack in her wall in "The Eleventh Hour".
- The Saturnynians, who fled their damaged world for Earth through a crack in "The Vampires of Venice".
Are there any potential in-universe explanations for why the cracks in the universe are not lethal to these few characters?