In כתב יד קאופמאן (complete manuscript can be found here in stunningly high resolution; basic information about the manuscript can be found here or in English here), in ברכות פרק א’ משנה א we have: תפלת הערב אין לה קֶבַע
Later in the same chapter, משנה ד we have: רבי אליעזר אומר העושה תפלתו קְבַע אין תפלתו תחנונים
So which is it, קְבַע or קֶבַע?
Here the difference is probably pausal vs. non-pausal. But generally there is a question of how to pronounce Hebrew words of this form, which include שבח and נפש.
Some people might be familiar with a dispute as to how to pronounce the word ושבחך in לדור ודור נגיד גדלך. Most pronounce is וְשִבְחֲךָ while others pronounce it וּשְבָחֲךָ. Baer in his Siddur Avodat Yisrael discusses this, and there are grammatical arguments for each pronunciation. It is discussed by רבי יוסף ברבי יצחק קמחי (father of רד”ק) in his ספר זכרון p. 22. It is also discussed by Shimon Sharvit in לשונה וסגונה של מסכת אבות לדורתיה p. 212, and by Nissan Berggrun in עיונים בלשון העברית p. 30. Many others discuss the question as well.
Regardless of which grammatical arguments are right and wrong, there are different reading traditions regarding many of these words. One of the more interesting words in the category is נפש which according to a Yemenite tradition is pronounced נְפָש (see Qesar – Hagayat Nefesh-Nfash be-Masoret Teman – Tema 5750). I will have much more to say about the word נפש as we go through the Mishnah, and especially about the meaning of the word in תנ”ך, for which you can check out the na”kh blog.
For those of you who pronounce it וּשְבָחֲךָ, you might consider being consistent and in ברכות קריאת שמע in the morning saying תִּתְבָּרַךְ יי אֱלהֵינוּ עַל שְׁבַח מַעֲשי יָדֶיךָ, and other examples. The consistency point goes also for those who say שֶבַח.
This post is very much an outline. Any treatment of this subject would take hours to research and write. I only have a few minutes. Blogging is harder than I thought it would be.