If you've had any contact with mechanical engineers/technicians - asking them to make you parts for something - you'll notice they'll write 'two off' for two parts OF something e.g. two screws, 'two off'. In everyday use this also manifests itself in the phrase 'x was a one-off' (it won't happen again). Until today I've always maintained that this is WRONG (so much so that I'll write 'two of these' on my mechanical drawings rather than 'two off'!!), but has become so widespread that it seems normal. I just looked it up in the OED and it's there! But I guess it's not the purpose of the OED to say what's wrong and what's not - it just documents language.
It really annoys me that I can't find some explanation for it - anyone have any ideas? Some justification for its use...

Can't help you. But you truly are a very deep thinker!

can't help you either, but I have seen a fair share of mis-spellings that have made me cry with laughter...

example:

I recently had some wrought iron gates on my house. The invoice itemised it as "raw tyon" !!!!!!!!!

It seems to me that everywhere I look, people consistently spell 'lose' (i.e. to misplace something) as 'loose' (i.e. not tight enough).

I have dabbled in carpentry and such like and using 'X off' to refer to quantity is not an error. It originates from foundry when products were cast off a mould so you would get '20 off that pattern and 30 off that pattern'. If something is unique it is a one-off which refers originally to a prototype and the original mould just being made from sand which was cheaper than producing a more substantial mould for something that could have potentially been crap, so the mould being made out of sand was only used once i.e. One-off. Subsequently we get the phrase 'they broke the mould with him' referring to someone who is very unique or a one-off

Today's dictionaries are composed from corpora totaling millions of words taken from everyday language use (eg spoken/written language, journalism, ephemera)in an attempt to represent modern English. This means that although some expressions may seem wrong, the modern dictionary's aim it to characterise the language as it is. Lexicographers are now descriptive as opposed to prescriptive. They are very wary of deeming a form as wrong. Prescriptive grammars (eg The King's English) document how language should be used.

I agree with what you say and I am astounded to learn that in a matter of years 'I would have eaten' is likely to be replaced by 'I would of eaten' due to current usage.

I hope that's a little clearer.

Cool Tricky. That makes me feel a lot better - I'll actually use it proudly now in my thesis
Apostrophe - do you reckon that language only evolves through mistakes then? (words always moving further away from their original meaning and becoming less and less clear) I can't imagine any scholars sitting around thinking of ways to improve the language...not that they could force the population to comply anyway.

hm i guess that is a bit language specific. german has recently had a major overhaul which was designed by language specialists, decided in high profile meetings, and now needs to be implemented everywhere. there was a multiple-year adjustment time but i believe that is now over. so all the rules i learned as a child don't apply anymore
but then, new words do constantly crop up in german, too. often they are derived from other languages, but become 'german' through their general usage.
french on the other hand has a specific institution to guard the 'purity' and correctness of the language, which is why they use french acronyms for stuff that is otherwise known in most of the world by its english acronym (onu instead of uno, sida instead of aids, etc.) and they 'invent' own french words for new stuff like 'computer' which in german is also a computer but in french an ordinateur.
i have heard that english is, compared to other languages, extremely usage-driven.

That's an interesting point, sue. I wouldn't say linguistic evolution occurs through mistakes, because mistakes are irregular slips. I would probably call it variation where a word's meaning changes over time - take the semantic development of 'gay', for instance. Language change is an interesting concept, especially in English where there are so many varieties.

Shani's very right, especially with the German spelling reform and with France's Académie Francaise. However, l'Académie Francaise isn't regarded with as much respect as it used to be and sees language variation from a purist's perpective, whereas with English we could say we have a much more tolerant interventionist approach to change.

Yeah, I guess it's not really true to say that words moving away from their original meaning is bad or somehow degenerative - what they originally described may no longer even exist! It's only natural that they are adapted to whatever is current in society. But that's all linked to the MEANING of words - what about grammatical/spelling changes?
Shani, I do vaguely remember being told at school how the Germans kept changing their minds about the β (or double s). Interesting - I wonder have there been any recent (in the last century say) 'authoritative interventions' in Britain?

sue, the recent german reform was mostly about spelling. some words were 'germanized', that is, a new spelling was decided that hides the originally foreign language word, for example 'portemonnaie' which is french derived and means purse (porte is 'carry' and monnaie is 'coins' in french) is changed to 'portmonee'. if you read the latter following german pronounciation rules, you will make the same sound as when you read the former following french pronounciation rules. so the sound of the word is kept the same but the spelling is made german. there were lots of changes about when to use capital letters, too.
the double-s thing is funny... we don't have that letter in switzerland! so there are quite some spelling differences between german-speaking countries.
don't know about Britain, but would be very interested. apostrophe? or anyone?