If the vote is relatively close, it can be unclear when it is appropriate for a losing candidate to concede an election. On election night, pressures from a media looking for news to report, an opposition campaign anxious to declare victory, and one's own campaign unwilling to concede defeat if there is any hope of a last-minute turnaround all weigh on the decision of the losing candidate. Though a spoken concession does not necessarily deny a candidate office if there is a drastic reversal in the vote count, it does practically prevent demands for recount.
It is exceedingly rare (and would be highly embarrassing) for a concession, once issued, to be retracted; such an event occurred in the 2000 presidential election, when Democratic candidate Al Gore, Jr. telephoned Republican George W. Bush to concede the contest. Gore was apparently unaware of the close vote count in the state of Florida, and when he realized this, proceeded to cancel his concession address.
A losing candidate commonly offers a private concession directly to the winning candidate (usually by telephone) before any public announcement is made.