When a similar thing happened in the 2010 election in Britain, the main third party (the Lib Dems) was able to form a coalition with the Tories. Here, the figures don't add up. There have always been small parties here picking up the occasional seat, but they have never commanded enough support to threaten the big two, and nor did they yesterday. What has changed is the emergence of two new parties in response to the recent recession, and they drew significant support from those disenchanted enough with both PP and PSOE to move their vote.
To the left of PSOE is Podemos (We Can) a party which has evolved out of the anti-austerity street protests of Los Indignados. They won 69 seats yesterday. The centrist party Ciudadanos (Citizens) which started life as the anti-independence party of Cataluña, took 40 seats. A coalition is called for, of course, but even so, the figures still don't add up.
PP (123) and Ciudadanos (40) have a total of 163, 14 short of that elusive overall majority. On the other hand, PSOE (90) and Podemos (69) are 17 seats adrift. The only arithmetic that would work - PP plus Podemos - wouldn't work politically; it would be like the Tories forming a coalition with the Socialist Workers' Party.
Both groups will no doubt be looking to see who they could work with among the 28 seats held by the various minor parties, none of whom can offer enough seats from a single party. Under the Constitution they have two months to work something out. If they don't, then the King will have the authority to order fresh elections. We live in interesting times!