Su-min's dismissive characterization of Hee-yeon's grandfather as a seller of "unimportant" side dishes (banchan), when the old man is in fact the powerful Chairman of U&K Foods. The more formal Korean meals are, the more banchan there will be served, akin to European fine dining. Korean royal cuisine used a setting with 12 cheop. It marks Su-min as nosy (for caring in the first place what a coworker's grandfather does for a living), and her immediately making a face when Hee-yeon's answer proved neither impressive nor salacious also says a great deal about her character.
Hee-yeon being unable to control her temper when Min-hwan and Su-min tag-team each other with mean-spirited comments about Ji-won's eating habits make more sense with the reveal that the younger woman is a Big Eater who enjoys her food unreservedly.
As an only child, Ji-won evidently couldn't recognize Ji-hyuk and Hee-yeon's sibling dynamic, and jumped to the wrong conclusion that they were dating.
Ji-won having a Ripple-Effect-Proof Memory makes sense and is not just for the plot. Her conversation with Hee-yeon about her drunken mistake shows that she's the kind of anxious person who remembers and internalizes even the little things, so it stands to reason that baseline Ji-won would have a good memory in the first place. Furthermore, while she gets a lot of things right, Jiwon is neither infallible nor all-knowing about the past timeline - her mistaken assessment of Ji-hyuk and Hee-yeon's relationship being a notable example.
Many viewers reported being unable to take Min-hwan seriously (or even finding it hard to hate him), simultaneously because of his actor's comic role off Welcome to Waikiki and the Marry My Husband narrative going out of its way to make him look comically awkward, even when he's going through a Humiliation Conga or sexually harassing Ji-won. These choices serves to highlights something that's Truth in Television: abusers who look and act like Min-hwan (and for that matter, like Su-min too, but the double-faced mean-girl archetype is more well-known) do exist. Shallow people on the outside of the relationships will only look at Min-hwan and see an endearingly dorky boyfriend to a frigid and bewildering woman, or Sumin as a long-suffering, sweet-natured, sacrificial best friend to someone who doesn't appreciate her enough. Ji-won and her true inner circle are able to get past appearances and expose the rotten core.
