Sara's oppression of Hagar
In next week's parsha (לך לך) the Torah tells us how Sara gives her maidservant Hagar to Avraham and Hagar becomes pregnant. Hagar then looks down on Sara, Sara complains to Avraham and Avraham tells her, do what you want with Hagar. The pasuk (ט"ז,ו) then states ותעניה שרי ותברח מפניה Sara oppressed her (Hagar) and she ran away .
The Ramban in a very short but important comment states that Sara's behavior towards Hagar was improper and in fact an aveira. Avraham's acquiescense to her behavior was an aveira as well. The Ramban then says that this behavior/aveira had ramifications for the Jewish people as a whole, because of Sara's oppression of Hagar, Hagar's child, Yishmael, was given permission to oppress the Jewish people.
We see 2 important lessons from this Ramban.
1. The root cause of the Arab hatred towards us is Sara's oppression of Hagar (very similar to מעשה אבות סימן לבנים which the Ramban says many times in Bereishis)
2. The Ramban was not afraid to say that Sara (and Avraham) sinned here.
Note: many other commentators disagree with the Ramban and offer various explanations as to why what Sara did was not a sin (for a summary of these see the Artscroll commentary, in addition see the Netziv who directly argues on the Ramban).