Julian Sanchez names a pet peeve:
With a “weak man,” you don’t actually fabricate a position, but rather pick the weakest of the arguments actually offered up by people on the other side and treat it as the best or only one they have. As Steve notes, this is hardly illegitimate all the time, because sometimes the weaker argument is actually the prevalent one. Maybe the best arguments for Christianity are offered up by Thomas Aquinas or St. Augustine, but I doubt there are very many people who are believers because they read On Christian Doctrine. Probably this will be the case with some frequency, if only because the less complex or sophisticated an argument is, the easier it is for lots of people to be familiar with it. On any topic of interest, a three-sentence argument is unlikely to be very good, but it’s a lot more likely to spread.