March 1956: The accident which claimed the life of Alfonso, brother of Juan Carlos (present king of Spain), kills Juan Carlos instead.
Alfonso was the younger, smarter brother of the two (so much so, Juan Carlos was referred to as "Juanito" -"Little John"-)
http://en.wikipedia.org/wiki/Infante_Alfonso_of_Spain
http://en.wikipedia.org/wiki/Juan_Carlos_I#Early_life
So, Alfonso becomes heir, later King of Spain as Alfonso XIV.
A smarter king of Spain might turn out being a good thing for Spain and a bad thing for the Spanish monarchy... IMHO a good part of the popularity and success Juan Carlos has arises from personal charisma, rather than political
savoir faire.
Another alternative: in OTL, when that happened, Juan Carlos allegedly wanted to give it all up and become a monk. Let's assume he does so and renounces his rights publicly. He was 18 and a couple of months at the time.
He would have royally screwed the succession, since he's got only sisters left... and while Salic law would not apply, I don't think a Queen would have been as popular in male-dominated Spain of the time...
Granted, this last would require a big personality change. But it was a very traumatic event. To quote the works of expert in Royal House history, David Joseph D'Anjou Medici-Coburg(1):
"Little Fonz yesterday got killed
Franky Frank made a gift of the piece
black gift that did the black deed
Jay Kay held the cursed steel:
fools play with things that kill.
Hammerfall: a prince in exile
is news and a chalk outline
"Dear John" from the C.S.I...
photo flashes and a crown dies.
Jay Kay with a 186
bailing out of Estoril Beach
crimson blue on the palace floor
Must atone, be a king no more..."
(1) DJ D'Anjou MC, "Death in Sou'wesside" from the album "Bourbon House Represent!", edited by Gotha Records, 1985, Iberian Massive(2)
(2) I know, I know, I'm weird for a first post.