They had a doorway into the past. But it didn't just open on a specific place on a certain day. There was a sort of flutter, and it caused spatial drift and temporal spread. Two probes might go through together on the 21st-century side but come out miles and years apart on the Silurian side. So, when the second of two people going through a portal into the past is lost, the first feels an abstract guilt that eats into her pleasure at having traveled through time.
"A very amateurish production"