Unconsciously, you're thinking, "This door might not stop for my arm." In the span of half a second, you weigh the options of saving the runner time in exchange for arm, or sparing your arm and leaving the runner for dead out in the lobby. Now let's assume you're a rational person. You choose your arm... Good choice, because you'll need it to fend off other people in the elevator trying to beat you up for not stopping the door.
"Why didn't you help that guy?"
"Well since you didn't stop the door, I guess you won't stop my fist either!"
Things are looking grim. You need an alternative. Plan B is to do a quick wave of the arm through the door in hopes of triggering the sensor. Save the runner, and your arm - a perfect outcome.
You forgot reputation. Everyone, including the sweaty person that just entered, would begin to make fun of your dooraphobia.
"What was that? Are you afraid of the door?"
"Better watch out, or the elevator monster is going to get you!"
I'd take the black eye over that.
So what now?
The doors are inches from closing, when you look down and see that glowing "Open Door" button. That's it! The safe, easy option to save the day. But there's no time! So you throw your arm down taking no heed to the accuracy of your hand.
Almost there... and... oh shit...
You hit the "Help" button. Doors closed, the elevator rises sans runner, and a faint voice is heard throughout.
"Hello. Do you need assistance?"
"No, sorry I was trying to hit the button to open the doors, and missed."
You can practically hear the facepalm coming from the guy in the control room. Everyone else in the elevator just looks at you in disbelief.
No words... just disbelief.
On a not-so-lighter note, I was getting off the elevator and this blueberry of a lady starts trying to get on. Now I may be way off here, but isn't the correct elevator etiquette to let the people off first? And it's not like she was a thin woman entering on one side, while I comfortably exited on the other. This woman was round. More spherical than the Earth, with only slightly less mass. There was no room to exit, and she wasn't stopping. I had to actually back up, to let her in!
If that was all that happened, I wouldn't be writing about it.
But get this. Get THIS!
Just out of politeness, I say excuse me. It was genuiine sounding, but deep down it really meant, "Excuse me... is what you should be saying to me right now!". Needless to say, I was expecting some sort of apologetic gesture. An "excuse me, sorry, pardon" are just a few of the options this woman had to satisfy me.
No... Instead I get an "Mmmhmm".
"Mmmhmm"??? Like "Mmmhmm. I will excuse you, mmmhmm."
I hope that elevator broke, so she had to take the stairs, and while taking the stairs, she fell back, and rolled all the way to the bottom. She would bust through the exit door so fast, witnesses would think they're in a cool aid commercial. Of course there would be civilian casualties, due to the doors acting as projectiles after actually breaking off their hingles, but it would probably be a net gain to society. This woman was like 5 negative people.