Pertwee famously mispronounced the word "chitinous" in Doctor
Who: The Green Death. A fan wrote in to the
producer, Barry Letts, telling him: "The reason I'm writin'
is how to say 'chitin'."
Pertwee pronounced the "chi-" part of the word as you would
say it in "chip". He should have pronounced the word to sound
like "kite-in-us". How can a scientist as brilliant as the
Doctor make such an embarrassing mistake?
in mind that in most of the Doctor's adventures, even though
we hear him speaking in English, he is probably speaking in
Gallifreyan. The TARDIS's telepathic circuits translate alien
languages for the benefit of its passengers and those with
whom they interact.
In several Pertwee stories, including The Green Death,
we see the Doctor travelling many miles from where the TARDIS
is located. This could lead to one of two effects:
1. The Doctor's distance from his craft could cause difficulties
for the telepathic circuits, leading to errors creeping into
the translations that people hear as the Time Lord speaks.
2. If the Doctor were to move completely out of the TARDIS's
range, the translation facility might cut out altogether.
Hearing those around him speaking in English, rather than
having the words translated into his own native tongue, the
Doctor would quickly realise what had happened, and so would
switch from speaking in Gallifreyan to speaking in English.
We know from stories in which the TARDIS is apparently destroyed
or its translation facility rendered inoperative (such as
Mind Robber, Frontios and The
Christmas Invasion) that the Doctor can speak
very good English. However, his language skills might not
extend to the correct pronunciation of specialised terms such
Either of these eventualities would explain the slip-up in
The Green Death.