This was a few decades ago but we had 2 tiers of weed outs. The first was the intro classes that most everyone took their freshman year. Then there was another wave that was major specific and tended to show up 2nd semester of sophomore year into 1st semester of junior year, essentially the point where coursework was shifting almost entirely to be major specific.

So for instance, one of the notorious freshman weed out classes was a class that *every* engineering student took. But then each individual engineering major had their own handful of pain in the ass classes that showed up as the engineering curriculum diverged.