Some time in the early 20th century, a fire occurred in a crowded theatre. Although the exit doors that were unlocked were not blocked on the outside, the panicked attendees could not get out of those doors, and hundreds of people in the theatre died from the fire.
What happened to trap everybody in the theatre? What happened differently from then on?
What was the cause?
Based on what Marin Takanov already said, I would identify the main negligence, which led to the disaster, in the lack of outward-opening doors! In fact, panic generated a huge crowd: people in front of the doors couldn't pull the handle to open the doors because the huddle was pushing behind them!
How was this problem fixed?
Simply replacing inward-opening doors with outward-opening ones, so that you have to push (not pull) the door in order to open it.
I think the answer is
Iroquois Theatre fire
http://en.wikipedia.org/wiki/Iroquois_Theatre_fire
The reason:
The people who were near the doors, couldn't open it because they had to pull it and in the same time the crowd were pushing them to the doors, that's why people were trapped and died. This explain "Although the exit doors that were unlocked were not blocked on the outside, the panicked attendees could not get out of those doors, and hundreds of people in the theatre died from the fire"
What changed since then:
tl;dr
1)The Iroquois fire prompted widespread implementation of the panic bar - http://en.wikipedia.org/wiki/Crash_bar
2) A second result of the fire was the requirement that a fireproof asbestos curtain be raised before each performance and lowered afterward to separate the audience from the stage.
3) The third result was that all doors in public buildings must open in the direction of egress.
There's a psychological phenomenon whereby people are more likely try to exit through the main entrance, or the one through which they entered the building, rather than use fire exits. This can cause significant and potentially life-threatening delays in evacuation.
This is affected by factors such as:
Presence / absence / thickness of smoke (absence of smoke = lack of urgency = people tend not to use fire exits)
Signage / wayfinding aids (e.g. low-level aids to finding fire exits)
Familiarity with the building
Group behaviour - which way is everyone else going? (and in particular which way are the other the members of a close group such as a family, going?)
There are other factors at work, such as familiarity with the escape route: research suggests that people are often more likely to use a familiar exit that is further away than an unfamiliar exit nearby – again increasing evacuation times. This was often attributed to panic, although social scientists now seem to agree that seemingly-irrational flight behavior to an exit is perfectly rational: they are rationally seeking out an exit with which they are familiar. Group dynamics also plays a part; how a number of people, in different states of fear, influence one another. Evidence suggests, for example, that people evacuate buildings alongside colleagues with whom they have an emotional attachment In a shopping centre that social dynamic may be familial, with parents trying to control small children. How does the family group react, when each member is instinctively responding in a different way?
Fire Dynamics & Human Behaviour, AJ Welch, 7 Jan 2015
People generally exit through the main entrance without the presence of smoke; in the presence of smoke they are more likely to use the fire exit, and low-placed signs facilitate the use of the fire exit nearest the evacuee. Further familiarity with [the building] was related to exit strategy; with less experience, people are more likely to exit via the main entrance, not the fire exit.
The Oxford Handbook of Environmental and Conservation Psychology, Susan D. Clayton, p.56
People were so accustomed to opening doors by pulling on them, that the frenzy overrode the ability to reason calmly. They thought the doors were locked and panic spread. Since then, exit doors open outward with a push bar so no knob is needed.
