The butterfly compensation switch was a control in Fifteenth Doctor's TARDIS and was located on the TARDIS consoles.
It was a blue switch on the panel facing the TARDIS doors. When the Fifteenth Doctor was getting used to his controls, he forgot to set the switch and so encountered the butterfly effect when Ruby Sunday stepped on a butterfly. This led to the creation of an alternate timeline that turned Ruby into Rubathon Blue of the 57th Hemisphere Hatchlings. The Doctor revived the butterfly, restoring Ruby to normal, and activiated the butterfly compensation switch to prevent it happening again. (TV: Space Babies [+]Russell T Davies, Doctor Who series 14 (BBC One and Disney+, 2024).)