The completion of the recount in the Costa Rican presidential election shows the lead for former President Oscar Arias having grown from 0.5% in the preliminary result to 1.1% now. However, the final result has not been declared, as allegations of irregularities still have to be investigated. Costa Rican law provides for a runoff between the top two only if the leader is below 40%, and there is so far no indication that Arias will fall below that threshold. (The runner-up, Otton Solis, was over 40%, but apparently has fallen just below and the two are now 40.9-39.8.)
I do not know whether Costa Rican law allows a re-vote, which should be distinguished from a runoff in that it would take place only in the case that a certain level of irregularity had been legally shown to have taken place. A re-vote, as I am defining it here, would also entitle all candidates from the original vote to run again. A runoff, on the other hand, is a race between the top two* from the first round, and is triggered not by irregularities but by the failure of the leading candidate to have reached a stipulated threshold (or, as I have argued, it could be a stipulated margin) in the first round.
The most famous case of a re-vote was, of course, Ukraine’s 2004 presidential election (which was actually a re-vote of a runoff). Ukrainian electoral law was already clear at the time that a re-vote could be called if the Supreme Court determined there had been irregularities sufficient to call the election into question.
* Some second rounds permit more than two, although I am not sure those should be called runoffs. French legislative elections are an example. I am not aware of any second-round rules that permit more than two candidates in popular elections for presidents, except for the former German Weimar Republic.
(UPDATE: Somewhat later, Jonathan Edelstein made me aware of a country with more than two candidates in its presidential runoff: The Comoros.)