Sa'd was a very high ranking companion and a shaheed. Prophet Muhammad took much more care during his burial than he normally would, yet he still chided someone who said that he would go straight to heaven.
Lesson: If you have bad akhlaq, even the prophet cannot help you against the consequences
Later, Prophet Muhammad proposed Zaynab bint Jahash marry his adopted son and ex-slave Zayd ibn Haritha. When the couple was unable to get along and got a divorce, the Prophet married Zaynab instead.
This broke multiple societal taboos:
1) Making marriage between the upper class and lower class seem acceptable and
2) Showing that the rules between father/son relations do not apply to adopted sons.

Lecture notes available at http://www.why-quran.org/?p=598.

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