In Season 5, Episode 8 "Lady Lazarus", Don sees Megan out of the office after she resigns. Immediately the elevator door closes on her, he calls another elevator. The door opens but there is no elevator car and Don carefully looks down the shaft before giving up and going back into the office.
I've watched the next two episodes and up to this point there is no explanation for this strange incident. What is the missing elevator car meant to represent, and/or why did the writers include it?