In the latest report (3:15 pm Costa Rica time - GMT -6 hours) the TSE has released preliminary figures with 88.447 % of the voting stations tabulated.  The tally shows Oscar Arias hanging on by a fingernail to the lead, with 591,769 votes in his pocket. Otton Solís has closed the gap throughout the day, and now has 588,519.

So, the difference at this point is only 3250 votes.

The TSE has found that 4.446% of the voting table’s summaries are inconsistent.  This means that for some technical reason the results report is not able to be included in the preliminary tally.  However the court will include them in the hand count, after validating the ballot package and its contents. This leaves 7.107% of the votes to be tallied in the coming hours. 

Projecting the results in hand, the apparent (preliminary) outcome will be decided by less than 1000 votes.  So the next step will be to hand count the ballots.  The TSE always hand counts the ballots, but in most elections the winner is declared statistically and the hand count is just a formality. In this case it will be decisive.