Hm, good points, looks like I need more sleep. Looking at the video again, a path is valid if two slashes follow each other, with the same pattern shifted by 1 the line below.

But I wasn't treating /\ as an invalid on the first line. Eg thinking of:

    /\/\/\/\
    \\//////
    /\\\\\\\ … and so on

(Using the /\ pattern, paths can go back up and come back down, so this requires a lot more thought than I instinctively put into it)