Due to a bug/trap as reported in http://debbugs.gnu.org/13361, here is
one bug in calendar that moves window-point to (point-min).

1. Emacs -q
2. M-x calendar
3. C-x o so that the *calendar* window is not selected
4. M-x calendar-redraw

When you C-x o back to the calendar window, your window-point is no
longer at today's date but (point-min).

Note also calendar-redraw is called when saving in the diary file where
it has a save-hook: diary-redraw-calendar; so it is not uncommon for
this annoyance to happen.