Robot clown (The Day of the Doctor)

At some point after fighting in the Last Great Time War, the Ninth Doctor rode his TARDIS through a supernova to save a robot clown, and then spent a week unsuccessfully trying to fully repair it. The robot clown, a therapy bot, then remained loose in the TARDIS. Much later, while still recovering from a recent regeneration, the Eleventh Doctor stumbled upon the robot clown while searching for a mirror in the TARDIS, and took a note to do something about it in the future.

Eventually, the Doctor and River Song took the robot clown to the TARDIS workshop and repaired it. Unbeknownst to the Doctor, River used the robot clown on him, erasing his memory of counting the children present on Gallifrey on the last day of the Time War. They then released the robot clown near a farming settlement.

The robot was eventually given to Cass Fermazzi when she was growing up. However, it did not work as intended and did not take away any of her memories, as it was already full of the Doctor's, which it kept "spewing out", leaving a profound effect on Cass and inspiring her to later in her life "fight for what's right" in the Last Great Time War and "never be cruel or cowardly." (PROSE: The Day of the Doctor)