Having just helped two students through dividing line segments in High school Geometry, there are two assumptions that Sal makes in the video, but did not explicitly reference. The two assumptions are: 

1. When figuring out change in X/Y, the order now matters. In algebra 1, students could find the slope of the line using either point first, as long as they were consistent. In line segments, change in X/Y needs to consistent with where which point they will move from. In should be calculated as Endpoint - Starting Point (i.e. C-A in the exercise) and not vice versa.

2. There are two separate ways Sal uses ratios in the two videos. Although he explains it in both, he never clarifies that there are TWO methods. For diving line AC into segments AB:BC, then students must ADD the total ratio. So If AB:BC is 4:1, then there are five total parts in the final segment and the student will use 4/5 of the total change of X/Y. If dividing AC into AB:AC, then students DO NOT ADD the ratio parts. So if AB:AC have a ratio of 3:5, then students will use 3/5 of the total change of X/Y. 

I don't think a new video is needed, just a PDF summary page clarifying these assumptions. Anyway, the clarification helped my students. ~ Nathan