Sahih Bukhari, volume 6, book 61, hadith 513 (emphasis mine):
Narrated 'Abdullah bin 'Abbas:
Allah's Apostle said, "Gabriel recited the Qur'an to me in one way. Then I requested him (to read it in another way), and continued asking him to recite it in other ways, and he recited it in several ways till he ultimately recited it in seven different ways."
Since I don't know of seven different versions of the Quran, this seems to be in conflict to the belief in immutability of the Quran. The hadith should be held authenthic at least for Sunnis, so ruling it out is not an option. It is also supported by other hadiths, like the next one, which specifies that at least Surat Al-Furqan was allowed to be recited in several different ways, making explanation by re-revelation problematic. "Quran" in this context doesn't necessary mean the whole book, but it still seems that at least few verses should be included in more than one wording.
