// use heap memory as many modern systems do
    char *line = malloc(MAXLINE);
    char *longest = malloc(MAXLINE);

    assert(line != NULL && longest != NULL && "memory error");

    // initialize it but make a classic "off by one" error
    for(i = 0; i < MAXLINE; i++) {
        line[i] = 'a';
    }

So, you create something that does not fulfill the C library invariant of what constitutes a "string", and then pass it to a copy function that assumes this invariant? It isn't a fair thing to do, and frankly, I doubt it many beginner programmers care about things like this. Yes, they may run into such a "defect" and be very miserable for a while, but that will just teach them about debugging, and most important, invariants.

Zed, I appreciate your work, but if this is the direction you'll be taking with these articles, then don't bother.