When plans backfire
Rashi's comment on the verse about the nesi'im's contribution is to quote Rav Nathan: "What did the princes see that drove them to donate for the dedication of the altar in the beginning but not for the mishkan? Their plan was for the general population to bring whatever they can and then to bring whatever was missing. However, as the people brought everything, there was nothing left. So the princes brought the shoham stones as their personal contribution."
My grandfather observes that the letter missing from the word nesi'im here is a yud, the smallest letters that hints at humility. That is to indicate that the nesi'im had intended to have the mishkan attributed to their names, for a mitzvah is attributed to the one who completes it. In this case, they were missing zrizus for the mitzvah and so a letter was missing from their names. Consequently, they learned zriszus and came to donate for the mizbeach at the beginning.
Related post: http://kallahmagazine.blogspot.com/2015/10/dvar-yehudah-parsha-points-from-my.html