Af

A verse in Behukotai says:

וְזָכַרְתִּי, אֶת-בְּרִיתִי יַעֲקוֹב; וְאַף אֶת-בְּרִיתִי יִצְחָק וְאַף אֶת-בְּרִיתִי אַבְרָהָם, אֶזְכֹּר--וְהָאָרֶץ אֶזְכֹּר

The Pesikta Zutrata says:

ביעקב לא נאמר בו אף לפי שהיתה מטתו שלימה

meaning that the word "Af" was not said regarding Ya'akov Avinu since his bed was perfect. No Yishma'el or Esav descended from him, which would be a cause of "Af" (anger) before Hashem.
The midrash is saying that even though the word "Af" here is being used to mean "even", the second meaning of the word "Af" - anger - is considered to be so bad that Yaakov Avinu's name should not be associated with it due to the perfection of his seed.

We find a similar idea in the Sephardic Birkat Hamazon. In the "רצה והחליצנו" prayer that we say on Shabbat, early Nus'ha'ot would have:

ואף על פי שאכלנו ושתינו חורבן ביתך הגדול והקדוש לא שכחנו

Later editions did not want to have the word אף (or any final-phey for that matter) in the Birkat Hamazon, so the Nusah was changed to:

והגם שאכלנו ושתינו חורבן ביתך הגדול והקדוש לא שכחנו

The Zohar says that "Af" is associated with one of 4 Kelipot we try to keep away from our prayers. See p. 165 here.