As I see it if declarer calls for a card which is completely irrational in a bridge sense

For example declarer playing in hearts has two black singletons in own hand

A spade is led and dummy has AK in the suit

Thinks "yippee I can discard my club loser now"
and calls for a CLUB

I would allow that one to be changed  

thanks nick, but would not that be a case of just follwing suit?

I was thinking of:

declarer is playing in hearts and has 2 losing clubs in dummy and also in their hand, singleton diamond in dummy and AKQ diamonds in hand and calls for a club from dummy (meaning to call for a diamond)

Is this careless or allowable?

Yes I left a bit out
I meant to say "after winning the first spade" declarer calls for a club
which makes it essentially the same as your example

Both think they can get rid of a club
and "club" gets stuck in the brain so they absent-mindedly call for a club lead

The essential part of the law is "(it may be changed) if declarer incontrovertably never intended to call for the card they did"

That wording means the declarer never gets the benefit of the doubt

An example of NOT being allowed to change would be
declarer leads towards dummy and calls for the ace without noticing LH opponent has trumped it

That fails the "never intended to call for that card" condition

Also (deviating a little) if a card is played (rather than called for) in is never allowed to be changed (unless it is illegal of course)
A number of people don't realise this

thanks giles.  how would you rule on the example I gave in my original query.

The applicable law (45 C4 (b)) says "a change ... may be allowed after a slip of the tongue but not after a change of concentration"

I would allow a change, but advise the opponents of the right to appeal 
Let the appeal committee cogitate over the semantics

If the opponents feel comfortable seeking an adjustment in this situation good luck to them