If you want to be really picky here then you have to consider the legal obligations faced by you and by Cambridge. A conditional offer is a legal offer and can be terminated in one of six ways:
1. By effective acceptance of the offer - applies to you
2. By rejecting the offer - applies to you
3. By revocation by the offeror prior to acceptance - does not apply to you as you are in the course of performing the offer's terms. As long as you do not vary the terms of the offer, the offer will remain open until any stated time expiration occurs.
4. By lapse of time - an offer ceases to be legally binding when an express length of time expires, or when a reasonable length of time expires. Basically you have until some point later in the year to accept the offer - April/May probably, unless Cambridge have stated otherwise. (I believe that Colleges ask you to indicate whether you will be accepting the offer - but this isn't binding and you don't have to accept any offers until you have received replies from ALL the universities to which you have applied).
5. By failure to fulfill a condition - this is the AAA offer, and unless you fulfill these requirements then the offer can be treated as repudiated (terminated) by Cambridge as you are in breach of its terms. If, therefore, they have stated 'AAA at A level in X, Y, and Z, and completion of your A level in W...' then you must do EVERYTHING stated to ensure your place. However, if they have said 'You are expected to complete all A levels not included in your conditional offer' then this is a wish, but not an obligation. However, I expect that somewhere on Cambridge/UCAS paperwork it states that conditional offers are made on the basis of all written and oral evidence given - and if you were found to have altered any exam results etc, then they would probably be entitled to refuse to accept you, but this is unlikely.
- The above point depends on so many things - at what point they find out your grades are incorrect, the extent of the misrepresentation (i.e. if you say you got an A and got a B then this is not likely to have much affect on your application should you achieve AAA. However, in the unfortunate circumstance that you get AAB and they know your grades were wrongly reported, they may look at other candidates more favourably in the Summer Pool) and loads of other aspects.
Oh and what you said about not realising the mistake until you got your certificates - sounds like a load of b*ll*cks to me as most people applying now have had their GCSE certificates for ages, certainly way before the Oxbridge deadline. Whether it is true or not, it doesn't sound very convincing and there is probably some legal loophole where they could revoke the offer, but it's pretty unlikely. The effect(s) of the mistake on the outcome of your application are negligible.
6. By death - hopefully this won't happen!
Overall, you must fulfill ALL of the terms of your offer in order to ensure a place. What would help is if you posted the exact wording of your offer and whether or not the mistake about grades is true or just something you were wondering. In any event, I would say the best thing is to let Cambridge know of any details like this ASAP, because if you are one of the unlucky people whose grades are checked and verified by UCAS then you could find yourself disappointed. Hope that helps. x