Thanks to Kenny Z for his explanation on the ruling in this situation.  Two
points:
 
1) As Kenny said, there is the human delay factor, so even though the goal
  lights are tied in with the clock, it is possible for the puck to go in
  and the light to go on, but the horn then sounds after the goal but before
  the timekeeper sees the signal and attempts to shut the clock off.
2) It isn't always true that the buzzer sounds when the clock hits 0:00.  At
  many rinks, there is actually another second between the clock changing
  to 0:00 and the buzzer sounding.  At these rinks, the clock changes from
  20:00 to 19:59 at the exact moment the clock is first started.
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Mike Machnik        [log in to unmask]       [log in to unmask]