The fallacy of hasty generalization occurs when an arguer draws a broad (or even universal) generalization from a sample that is too small to justify the breadth of that generalization.

Here is an example:

Jason said, "I looked for fish in the cafeteria on Monday, Tuesday, and Wednesday and didn't find any. So I suppose the cafeteria just never serves fish."

Jason is guilty of hasty generalization, because three "fishless" days in the cafeteria is not a large enough sample to generalize about it's lacking fish all year long. For all Jason knows, the cafeteria might serve fish every Friday, for example.