After a Virginia, USA school decided to sell their used Apple iBook laptops really cheap, panic and stampede ensued, as humans exhibited their worst behavior. The local newspaper, the Times Dispatch also has an article and even a slideshow showing people pushing and trampling on falling fellow human beings. Slashdot also has some comments, some from those who attended.
This kind of behavior is rare in Western societies, but is part of daily life in some third world countries. It is often attributed there to several factors, like overpopulation, lack of resources, limited services, ...etc.
Imagine a Western society that experiences a shortage of some vital resource, such as fuel, bread or water, would this uncivilized behavior take over? Would human selfishness cause some to get that resource at all costs, even if it means harm to others? Would "harm" here include depriving others of this resource, or even trampling them?
This is just one indication that common decency and civil behavior is just a thin veneer over a more animal-like core of human being that gets out whenever it has the chance.
A mob is the most dangerous thing one can get caught in. A mob has no leader, has no logic or reason, and no sense of right or wrong or morals. People who as individuals would not do bad things will certainly do them if they are in a mob where responsibility is diluted and spur of the moment actions happen.
From the above, it is clear that mobs are not limited to hundreds or thousands of people in the street chanting slogans because of a political or religious reason. Some mob behavior can be exhibited in smaller groups (e.g. a group of soldiers in war, who think there is no law, and are encouraged by others behaving badly, and again diluted responsibility).
We also see mob behavior in situations such as sports fans going on a rampage and destroying property and even killing people. Some Canadians, who are normally very nice people can turn into mobs because of things such as hockey. Another case of unruly mobs is when groups want political, social and economic change, such as the Seattle World Trade Conference