ElanHasson.com

It's certainly common at Microsoft,
and probably common at many places, that a meeting runs over.
The next group who has booked the room gathers outside
waiting for the previous meeting to wrap up.
Sometimes they wait timidly outside the door,
and the group inside never realizes that they are running over.
Late meetings have a cascade effect on the rest of the day,
and not just for the specific conference room.
Of course, if the 9am meeting runs late, then the 10am meeting in
the same conference room will start late and consequently run late.
But it also makes late
the 10am meeting that one of the 9am participants
needs to attend.

I learned this technique from a colleague:
Barge into the room at the official start time,
even if there are remnants of the previous meeting still in the room.
Depending on my mood, I will then employ one of these two tactics:

• I'll take a few steps into the room, then look around and
  say, "Sorry, I must be in the wrong room," and leave.
  
• I'll pull up a chair and sit down and apologize for being late,
  on the assumption that the meeting started without me
  and the people in the room are the actual meeting attendees.
  I might even start introducing myself,
  acting as if I was added to the meeting roster as
  an outside participant.
  

The second technique works only if the room is filled with
people who don't know you.