If the Targum on the Torah written by Onkelus was given at Har Sinai, How can Rashi say that he made a mistake?

If the Targum on the Torah written by Onkelus was given at Har Sinai, How can Rashi say that he made a mistake?

See Gemara מגילה דף ג ע"א Megilla 3a:

ותרגום של תורה אונקלוס הגר אמרו? והא אמר רב איקא בר אבין אמר רב חננאל אמר רב מאי דכתיב (נחמיה ח, ח) ויקראו בספר תורת האלהים מפורש ושום שכל... ויבינו במקרא ויקראו בספר תורת האלהים זה מקרא מפורש זה תרגום

The Gemara asks: Was the translation of the Torah really composed by Onkelos the convert? Didn’t Rav Ika bar Avin say that Rav Ḥananel said that Rav said: What is the meaning of that which is written with respect to the days of Ezra: “And they read in the book, the Torah of God, distinctly; and they gave the sense, and they caused them to understand the reading” (Nehemiah 8:8)? The verse should be understood as follows:... “distinctly,” this is the translation...

This indicates that the Aramaic translation already existed at the beginning of the Second Temple period, well before the time of Onkelos.

The Gemara answers:

שכחום וחזרו ויסדום

The ancient Aramaic translation was forgotten and then Onkelos came and reestablished it.

The Gemara in Kidushin 49a says: ר' יהודה אומר המתרגם פסוק כצורתו הרי זה בדאי והמוסיף עליו הרי זה מחרף ומגדף:

Rabbi Yehuda says: One who translates a verse literally is a liar, [since he distorts the meaning of the text], and one who adds his own translation is tantamount to one who curses and blasphemes G-d...

Rashi explains: המתרגם פסוק כצורתו - שבא לחסר תוספת התרגום שלנו לומר לא אתרגם פסוק זה אלא כצורתו: One who disagrees with the additions added in our targum's translations, and he says "I will translate this passuk literaly"...

והמוסיף עליו - שבא לומר הואיל וניתן רשות להוסיף אוסיף גם אני בכל מקום שארצה:

And one who adds to it, and claims that that "If additions can be made, I too will make my own additions wherever I please".

And then Rashi adds: ואונקלוס כשהוסיף לא מדעתו הוסיף שהרי בסיני ניתן אלא שנשתכח וחזר ויסדו כדאמרי'ושום שכל זה תרגום (מגילה דף ג.):

Onkelos Did not add anything of his own accord, for the Targum [of Onkelos] was given at Sinai, as the Gemara states in Megilla 3a that the targum was forgotten and then Onkelos came and reestablished it".

If so, how is it that in numerous places in his commentary on the Torah, Rashi quotes the Targum Onkelos and than says that his translation is a mistake? [3 examples out of many: Rashi on Beraishis 15:11, Rashi on Beraishis 43:2, Rashi on Devarim 17:5, and many more.]

Answer

According to some translations of Rashi, you have simply misread these cases.

As examples:

According to the translation of Rashi (Bereishis 43:2) found at Alhatorah (I'm literally too lazy to see which edition it is), the words

והמתרגם: כד ספיקו, טועה הוא‏

do not mean that Onkelos has made a mistake, but rather it means:

He who has the reading in the Targum כד ספיקו "when they had enough" is in error.

Same in Devarim 17:5:

המתרגם אל שעריך – לתרעב בית דינך, טועה

is translated as:

He who renders אל שעריך in the Targum by לתרע בית דינך, unto the gate of thy court, is in error