Dave Carr has just posted an ornithological treatise of a blog stating that the Bearded Tit is not a Tit at all. I commented about Welsh Rabbit (I know, I know - it should be Rarebit!).
What other phrases are patently not what they seem at all?
Bombay Duck?

French letter? Interestingly enough, the same thing is called an english hood in french (capote anglaise)...

Dutch cap?
I tried and tried, but even though I have been known as 'pin-head' I couldn't manage it

Likewise, I understand that French Leave is called English Leave in France.

Britain's private schools are known as public schools.

Of course there are scholarships and many of them are charities......

What about "Allo, Vera"

This isn't strictly to the point but the English language does have other peculiarities.

Putting 'in' on the front of a word normally reverses its meaning to the negative. Appropriate > inappropriate.
However flammable and inflammable mean the same.

Awesome presumably means creating some awe. Awful, should mean full of awe shouldn't it, but means the opposite.

The hood/soft-top of a car such as the 2CV is known as la capote, and they are often sourced in England, hence my car is covered by une capote anglaise. a spanner is une clé anglaise - dunno why - and when I crack jokes in French and they don't understand they shrug and say, ah, l'humour anglais (i.e. famously not funny), from which they wish to be saved, or preserved; and another name for a condom in France is préservative, which you won't find in food, one hopes.

piccadilly circus...never so much as a dancing horse to be seen (elephant and castle was another real disappointment [not to say that I had a non-booking in the first place...])

When I was about fourteen and rather whimsical I went all the way to Angel on the tube because the name sounded so beautiful . . . .

I once got conned into going into the flix to watch a film called "Black Leather Pants" which turned out to be about ski-ing.

The purple finch...it isn't (seems this rounded-off the thread back to John's tits ;0)