An example from this week's sedra:
The verse [Gen 25:1] says: And again, Abraham took a wife, and her name was Keturah, but every school child is taught to ignore the plain meaning of the words on the page , and to instead accept that Keturah was Hagar. Why? Because Rashi said it. And why did Rashi say it? For one reason, and one reason only : In verse 6 the word pilagshim is choser, ie: with no
Only, hold on: In the MT the word is malay (spelled with a
Rashi's comment here, like many of his comments, is based on a midrash, in this case one found in Berashis Rabba. This suggests that the Midrash's author also had a text with a
Alternatively, its possible that the midrash in Berashis Rabba isn't addressing the text at all. The midrash continues (and Rashi cites this elsewhere, though in the midrash the two thoughts are not seperated) that Hagar is called Keturah because her deeds were beautiful like incense (Keturah puns on incense) . Perhaps Hagar is construed by the midrash as a meritorious woman for the glory of her husband Abraham, and not because of an anamoly in the text. It would not be unlike Rashi to use a midrash written for one purpose to address another.
Given the Rashi does not cite every single Midrash , I thinks its reasonable to conclude that we are only aware that the midresh said Ketruah and Hagar were the same person because of an accident, the accident of Rashi's choser text.
 Alas, no school child is taught that the Rashbam disagrees with both Rashi and the misdrash and insists that Keturah was a third wife, and not Hagar redux. You're invited to complain about this to your local school authorities.
 I expect those of you who don't know how to learn Rashi are already yelling: "One reason only? What do you mean? It's in the midrash!" You are reminded that Rashi does not cite every single midrash. He picks and chooses among them for two purposes. As he says himself on Gen 3:8: There are many aggadic midrashim, and our Rabbis have already arranged them in their proper setting, in Breishis Rabbah and in other collections of midrashim. I have only come (1) to give the simple meaning (pshat) of Scripture, and (2) for aggadah that resolves the words of Scripture...
 Rashi also suggests that she was called keturah because "she "tied her womb" and did not mate with any man from the time she separated from Avraham" ["Tied" also puns on keturah.] This explanation is not found in BR, but is from Pirkei de R. Eliezer