Who is the author of the prayer "Nishmat kol chai" which is said on Shabbat, Yom Tov, and during the great Hallel at the Seder?
The Machzor Vitri(A talmid of Rashi who lived during 11th - 12th century) Hilchos Peasach 66 brings down that it is a mistake to think that peter wrote nishmas kol chai and if one says this he will have to bring a chatas when the third bais hamikdash is built.
In Siddur Avodas Yisrael pg 206 he brings two commentaries on who the author is of nishmas kol chai one from the 15th century that the author was Rabbi Shimon Kipah. The other source says it was Rabbi Shimon ben Kipeh and some say Rabbi Shimon Ben Shetach.
For an In depth discussion of this and many other such topics Listen to this Lecture http://www.yutorah.org/lectures/lecture.cfm/726352/Dr.%20Shnayer%20Leiman/Jewish%20Perspectives%20on%20Early%20Christianity:%20Toldot%20Yeshu
The lecture is an attempt to show that Toldot Yeshu is a much older text than it's first appearance in the 1700s. It explains there that when the Rashi says that the "Romani" have no language, have no alphabet, and other people write their books, he is referring to the idea that R. Shimon set up Latin as the Christian language, created the Christian Bible and otherwise created Christianity to make sure that Jews would not be drawn to it.
The Toldot Yeshu also claims that Nishmat Kol Chai was written by R. Shimon as well. (R. Shimon being Peter)
The Lecturer makes an empahtic point that this is just one Jewish tradition, but that there are others that disagree with it. His main focus was that the tradition is an old one.
Maybe Simon (Peter)?! See the whole conversation at http://www.globalyeshiva.com/forum/topics/curious-anecdote-discovered
or look at this: http://hebrewbooks.org/pdfpager.aspx?req=2585&st=&pgnum=285
cf. Mishna -Pesachim 10, 7
The author is St. Peter according to Rabbeinu Tam, though some say he only held that view due to pressures of the times.
A good analysis, with citations:
Whether factually accurate or not, we have established that there was indeed widespread acceptance of the notion that St. Peter had composed the Nishmat, Etan Tehillah and Ahavah Rabbah prayers. This appears to have been supported by Rabbeinu Tam and other Rishonim.
On the other hand this perception was vehemently challenged by Rashi, his student the Machzor Vitry and others who suggested the prayers may have predated the Common Era.
It is interesting to see that the brunt of the debate is played out between both Rashi’s grandson (Rabbeinu Tam) and his student (R. Simcha of Vitry).
In an attempt at reconciling the variant opinions, some suggest that there may have been two Shimons. One was Shimon ben Yona, the Apostle who was known as St. Peter who was viewed rather negatively by Rashi and the Machzor Vitry. The other was Shimon Kipah who was the righteous man who acted as the agent for the rabbis.
But that is just speculation.
In general the difficulty that confronts us is that;
“Since the names of the first paytanim (composers of liturgical poems) have been lost to memory, it has happened that the liturgical poems have been attributed to people who never thought to write them.”
This means that we may never have any real resolution to our question as to who the mysterious author was who penned these prayers.