Doctor Who: 10 Things Everyone Always Gets Wrong About The TARDIS
9. Why The Chameleon Circuit Was Never Fixed
The chameleon circuit is meant to change the TARDIS' outer shell to blend in with its surroundings. However, the Doctor’s TARDIS is locked in the form of a 1960s blue police box - but it turns out that this may be by design, not by chance.
The chameleon circuit did work during the Doctor and Susan’s first trip to Earth’s moon in the distant past, where the TARDIS disguised itself as a boulder. In other expanded media stories, it has also become a Corinthian column, a kiosk, a shed, and a Christmas tree. The knowledge that the TARDIS was stuck in its current form seemed to genuinely distress the First Doctor and Susan, but over time, he became nonchalant and actually liked it.
In the comic-book Hunters of the Burning Stone, it was revealed that the Eleventh Doctor traveled back in time to sabotage the chameleon circuit so that the TARDIS would be stuck as a police box, ensuring that the image of the TARDIS would stick in Earth's memory as a symbol of protection.
The broken chameleon circuit is usually made out to be an accidental flaw that the Doctor can't or won't fix - but actually, it might've been the plan all along.