Despite the existence of numerous editions (up to 14 at the beginning of the 20th century) of the Koran containing verses with different wording and subtle differences in meaning, Muslim apologists are adamant in their belief the Koran has been perfectly preserved in face of all countervailing facts.
However, it is notable the first generation of Muslims were never this convinced the Koran was perfectly preserved.
In his commentary on the Koran, Yusuf Ali notes in a footnote that one verse of the current Koran is missing an entire line that is present in the earliest "Qira'ahs" including the "qira'ah" of Ubay Kaab - the greatest of the early Koran memorizers and reciters.
In v33:6 of the Koran, which reads:
The Prophet is closer to the Believers than their own selves, and his wives are their mothers. Blood-relations among each other have closer personal ties, in the Decree of God. Than (the Brotherhood of) Believers and Muhajirs: nevertheless do ye what is just to your closest friends: such is the writing in the Decree (of God).
Ali writes:
In some Qira'ahs, like that of Ubayy ibn Ka'ab, occur also the words "and he is a father of them", which imply his spiritual relationship and connection with the words "and his wives are their mothers". [Abdullah Yusuf Ali, The Meaning of the Holy Quran, 11th edition, Note #3674]