After the description of the making of the Golden Calf, the Torah says that B’nei Yisrael offered sacrifices, and then “the nation sat to eat and drink, and they [then] got up to revel” (32:6).

The Midrash in Shemot Rabba 41, takes note of the word “vayeishev” (“sat”) used by the Torah in this context. While this term is itself innocuous, and there is certainly nothing criminal about “sitting”, the Midrash observes a pattern of the use of this term in Biblical narratives involving cardinal sins:

• The story of Migdal Bavel began when the people “settled” (“vayeishvu”) in the Shinar Valley (Breishit 11:2).
• In the story of the sale of Yosef, just before they sold Yosef as a slave, the Torah says the brothers “sat [or returned] to eat bread” (“vayeishvu le’echol lechem” – Breishit 37:25).
• The Torah introduces the tragic story of Ba’al Pe’or – where BNY engage ritually and sexually with the women of Moav – with the words, “Vayeishev Yisrael ba-Shittim” (“Israel settled in Shittim” – Bamidbar 25:1).

Here, too, in the context of the Golden Calf, we find the perpetrators “sitting” to eat.  The Midrash thus makes a statement, “Wherever you find ‘sitting,’ you find an offense [takala].”

אֹרַח חַיִּים לְמַעְלָה לְמַשְׂכִּיל לְמַעַן סוּר מִשְּׁאוֹל מָטָּה
The path of life goes upward for the wise, that he may depart from the nether-world beneath.

The Vilna Gaon, in his peirush on Mishlei, explains that a human being is called aholeich (walker), who must ascend from level to level, and if he doesn’t go up, then he will not stay in place, but actually go down.

Chazal saw within the pattern of “vayeishev” a warning against feeling toocomfortable.  While we must certainly take pride and satisfaction in what we’ve achieved, we must also be concerned about what we have yet to achieve.  If we fall into the trap of “vayeishev,” of staying comfortably in place without looking to reach higher and improve further, then we are prone to fall.

I think it’s interesting to note, that in each of the 3 instances listed by the Midrash, the “sitting” that took place, took place before the maiseh. Only in Cheit Ha’Eigel, did BNY sit after the sinful act of creating the Golden Calf.
[I encourage any and all suggestions for another idea as to the relevance, if any…]

Perhaps the Golden Calf differs slightly from the other 3 examples because of BNY unifying, but for a severely improper purpose, and something that emerges from looking at the sequence of events:
The people had Aharon create the Golden Calf, Aharon then delays by saying they will make a holiday on the following day (thus creating a break in the action), BNY arose the next morning, “sat” to eat and drink, and then got up to worship:

וַיַּשְׁכִּימוּ מִמָּחֳרָת וַיַּעֲלוּ עֹלֹת וַיַּגִּשׁוּ שְׁלָמִים וַיֵּשֶׁב הָעָם לֶאֱכֹל וְשָׁתוֹ וַיָּקֻמוּ לְצַחֵק
And they rose up early on the morrow, and offered burnt-offerings, and brought peace-offerings; and the people sat down to eat and to drink, and rose up to make merry.

The Ramban says on the passuk, that BNY sat as one in honor of the Eigel, presumably learned from the singular verb “vayeishev ha’am” instead of the technically correct, “vayeishvu.”
So what emerges, is that (i) the people were united as one, but for a terribly misguided reason, and (ii) during the pause, between creation of the Calf and the worship of the Calf, no one thought about turning the ship around. Their pause, and their lack of use of the pause, was critical. Their complacency, in this instance, was one of being so steeped in what they were doing, that nobody thought to re-evaluate their actions when given the chance.

The sin of the Golden Calf is probably the greatest and most costly collectively-perpetrated sin of the Jewish people. We went from being “k’ish echad b’lev echad” – as Rashi explains on the passuk “vayichan sham Yisrael negged ha’har” before receiving the Torah on Har Sinai – to being in the same one-ness for the exact opposite – for Avodah Zarah.
The contrast between the two was so terrible, that Hashem was ready to destroy us. Similarly, by Migdal Bavel, the people were also joined together, to build as one (as the Torah says: vayehi kol ha’aretz safah echad u’devarim achadim), and Hashem actually did destroy that entire generation. Unlike the Dor Haflagah, fortunately the BNY had a Moshe to plead with Hashem on their behalf, and avoid complete destruction.

Furthermore, in addition to Moshe’s teffila of the 13 Midos HaRachamim, the atonement for the Golden Calf was also the collection of the half-shekel for the Mishkan in next week’s parsha, Vayakhel. Part of the reasoning behind the half-shekel and Terumah for the Mishkan, was that Hashem wanted us to act, and not be passive, and to give, and not complain; but the actions taken and objects donated, had to be for a rightful purpose. We became builders of a Divine Sanctuary for the Shechinah to reside – instead of builders of a golden idol or a towering structure aimed at piercing the heavens.
In Vayakhel, the people come together for the correct purpose: to learn about Shabbos, to give their possessions, and to build a Sanctuary. They even give so much that Moshe has to put a halt to the giving, in 36:5-6.

When we shake off complacency, to join together and spring into action with the proper goals in mind, then we become worthy of the gifts of Shabbos, true cheirus, a Mishkan and hashra’as HaShechina.