Seffilun 27

 was a planet visited by the Thirteenth Doctor, Ryan Sinclair, Yasmin Khan and Graham O'Brien in the 67th century. It was located in a junk galaxy.

The Doctor confused Seffilun 27 with Seffilun 59.

A sonic mine was activated on the planet. The Doctor failed to keep it in temporal lock, and she was incapacitated with her companions until they were rescued by the Tsuranga medical ship. (TV: The Tsuranga Conundrum)