That would correspond to the following C:

    /* 280 */
    do_something();
    for (;;) {
      /* 290 */
      /* display chart in blocks of 18 lines */
      /* 400 */
      L = 0;
    }

when the correct code is this:

    for (;;) {
      /* 280 */
      do_something();
      /* 290 */
      /* display chart in blocks of 18 lines */
      /* 400 */
      L = 0;
    }

The bug is that the call to do_something() precedes the outer loop when it should be inside the loop.

Is that easier to debug in C than it is in the BASIC program? What's the difference?