The basis of virtually all logical errors is the assumption that "some" equals "all."
Do not assume that because you hold an opinion, the same is true of the rest of the world. If you write, "Nobody thinks smoking is acceptable any more," you will undoubtedly be assaulted by a mob of irate tobacconists. You must be aware of the circumstance and be more precise: "Nobody in my class thinks smoking is acceptable."
Politicians are especially adept at assuming that their position is that of the majority. If there is inconvenient evidence, in the form of election results or polls that contradict them, they may choose to invent a "silent majority" that actually supports them without ever saying so.
Deductive reasoning is a way of thinking that draws inferences from general statements or uses generalizations to apply what is true in one instance to what is true in another related instance. You must be careful not to assume that what applies to one situation always applies to another. A hasty generalization will draw a conclusion from an insufficiently representative source; for example, a survey of an English class might produce the information that 90had read Beowulf. One could not assume, however, that ninety percent of all university students had read it.
It was deductive reasoning that led to the ancient practice of letting blood as a "cure" for various diseases. If the patient is flushed, and her heart is beating fast, it can be deduced that she has too much blood in her body: pass the leech.
If a generalization does not stand up to the question, "Can you prove it?" think twice about using it.