Sherlock Season 4: 20 Easter Eggs & References From The Six Thatchers
14. Arresting A Jellyfish
Yet another brief case sees Sherlock and John return to Baker Street, laughing that the culprit of some unknown murder turned out to be a jellyfish. They are then interrupted by Mary going into labour.
Holmesians will recognise this as a nod to The Adventure of The Lion's Mane. In that story, a man dies in agony while out fishing. It turns out - spoilers - that he was stung by a type of jellyfish known as the Lion's Mane.