Unrepresentative Sample

If the means of collecting the sample from the population are likely to produce a sample that is unrepresentative of the population, then a generalization upon the sample data is an inference committing the fallacy of unrepresentative sample. A kind of hasty generalization. When some of the statistical evidence is expected to be relevant to the results but is hidden or overlooked, the fallacy is called suppressed evidence.

Example:

The two men in the matching green suits that I met at the Star Trek Convention in Las Vegas had a terrible fear of cats. I remember their saying they were from Delaware. I've never met anyone else from Delaware, so I suppose everyone there has a terrible fear of cats.

Most people's background information is sufficient to tell them that people at this sort of convention are unlikely to be representative, that is, typical members of society.

Large samples can be unrepresentative, too.
Example:

We've polled over 400,000 Southern Baptists and asked them whether the best religion in the world is Southern Baptist. We have over 99% agreement, which proves our point about which religion is best.

Getting a larger sample size does not overcome sampling bias.