The Twelfth Doctor created a hologram ghost of himself to trick the Fisher King.
The ghost appeared in the lake unto Clara Oswald just after the Doctor bid her a temporary farewell.
This caused a younger version of the Twelfth Doctor to believe he was dying and that it was fixed. Eventually causing him to create the ghost in the first place.
This fell under what the Twelfth Doctor described as a "Bootstrap paradox". (TV: Under the Lake / Before the Flood)