based on the gmatprep tests
- it is possible to get 3 wrong questions and still get 51 in the q section... (it happened to me...) however myfirst mistake was at question 29...
i don't know how it is in the real GMAT.
also - i'm not sure it is impossible to get 52 in q and v, it is just highly unlikely.... it depends mainly on the level of questions in the database and those chose for you. there are probably very few questions aiming at discerning 51 from 52, and i guess that if you do even one mistake during the test the likelyhood of getting such question (and then getting it right) is very very low.
to the best of my knowledge there is no "decision" that 51 is the maximum, but it is a result of the amount/level of questions in the database.
pelihu is right saying that once 51 gets too crowded we'll see some 52s. it is so because once 51 gets crowded more people will get to answer hard questions, which will put some pressure in adding more questions discerning people at that level, which in turn will increase the likelihood of people getting 52. but i don't think it is a matter of changing the formula or changing the algorithm or an aware decision regarding maximum score.