On the top of Ktuvot 87b is the end of Mishna 8 in this chapter. The Mishnah brings a statement of Rabbi Shimon. On 88a the gemara begins to relate to this statement, and in the end Rav Papa says that Rabbi Shimon relates to Rabbi Eliezer in Mishna 4. Abaye attributed the statement of Rabbi Shimon also to Mishna 5.
Whichever is accepted as being correct - the question arises: Why did Rabbi Yehuda HaNasi put the statement of Rabbi Shimon so far out of place?
One can see that the Tana Kama of Rabbi Eliezer in Mishna 4 is anonymous, and after that we have a multitude of Mishna without attribution to any Tana until the statement of Rabbi Shimon.
-Could it be that all this Mishna is an early Mishna: it was part of the Girsa , which was learnt by heart and which was well known - before Rabbi Shimon made his statement; Rabbi Yehuda HaNasi did not want to disrupt the well-known text, so he added the statement of Rabbi Shimon at the end?
It could be. I feel that it is more likely that the Tana wished to complete the whole issue of Shevu'ah, to which Rebbi Shimon relates. Although he only argues on the earlier Tana, his Din relates to all the Mishnayos.