In the consistency graph, the first day the task was skipped (10-12)
appears in green (org-habit-ready-face) on the graph. The second day
(10-13), when the task was overdue, appears in yellow
(org-habit-alert-face). If I am reading the manual correctly, I would
expect this second day to be red, since the task is overdue on the
day.
(See the attachment "graph-1.png").
I believe I've found another issue with the graphs. If a task is
completed twice on the same day, it prevents all subsequent days from
appearing on the consistency graph.