Electoral system where the candidate with the largest block of votes automatically wins. Fair if there are only two candidates, but with more than two usually results in a situation where more people voted against the winner than for. Over many seats, results such as the UK Labour party's 1997 victory can happen: 170 seat majority in a ~650 seat house but under 45% of the popular vote.
Some systems get around this by having a run-off between the top two picks. The failure to do so when it appeared that the incumbent lacked the majority has led to the current upheavals in Peru.