As a Customer Service Manager at DFW for the world's largest airline,
I was out in the operation helping board an oversold flight on a
busy summer afternoon a while back.  I remember we had a lot of
standbys that day (both employees and their parents, spouses, partners,
children, etc.) and frankly, chances looked pretty slim for most of them, to be honest.

After soliciting volunteers (who then joined the throngs of standbys
crowding the gate area) I started the boarding process.  Soon the
lounge cleared out to only those passengers and standbys waiting
for a seat.  We released the seats of the no-shows about 10 minutes
before departure time and discovered we had a handful of seats to
dole out in the order on the priority list in the computer.  We
handed the first seats backed to the volunteers, then did some juggling
for the couples trying to get seated together, and finally came
down to accommodating one revenue passenger who had missed an earlier flight.

It was now about 3 minutes before departure and I had very little
time left to get that aircraft door closed for an on-time departure.
I told the standbys that we were sorry, but there were no more
seats and I'd be right back to 'roll' them to the next flight. 
Immediately one man jumped up and accosted me at the jetbridge door
(he was really far down on the standby list if memory serves me
well.)  "Whadda ya mean there's no more seats - I WANT MY SEAT NOW
before this plane leaves," he bellowed.  "I'm sorry sir, but you're
standby and there are no more seats on this flight, I replied."
He looked shocked and replied angrily: "Look, I've stood by for
over an hour, and now I want my seat so I can get on this flight!" 

Luckily a fellow agent ran down to button up the flight, while I
took him aside for a stern discussion.  As it turned out, he was
an employee's brother who hadn't grasped the whole concept of standby
as it was explained to him.  He actually thought it meant waiting
until departure time to find out what seat assignment he'd have
on the flight.   And for that he thought he could fly free?!!  When
I finally got through to him, he became totally irate and just said
"I'm never flying this outfit again!"

Sometimes you've just gotta laugh and move on to help the next genius in line.

From: Anonomys

I was collecting boarding passes on a very full Los Angeles flight. A passenger had too many carry on bags and I told him that he would have to check one. He began to argue, using all the usual cliches "I fly all the time" "I always carry on three bags" "I'm a premier flyer" He eventually figured out that he wasn't going anywhere unless he gave up the bag. I was apologetic, trying to explain that the flight was full, and I was just enforcing the FAA policy.

He said he was going to write a letter to my supervisor. I chuckled to myself because I was sure that my supervisor would be happy to hear I was enforcing the carryon baggage limits on a full LAX flight on a Friday evening, since she had just written a briefing item saying we needed to have more enforcement. Angry that he couldn't intimidate me, he started down the jetway after I handed him the claim ticket.

Suddenly his briefcase sprung open, it's contents scattering all over. "How is that for Karma" the next passenger in line laughed as I took his boarding pass.