"Serendipity is looking in a haystack for a needle and discovering a farmer's daughter." -- Julius Comroe Jr.
Every decision and subsequent course of action has unintended consequences. Some times these effects result in positive discoveries (serendipity); other times they have negative reactions (blowback).
My favorite example of serendipity is Louis Pasteur's discovery of the vaccine. Pasteur was studying chicken cholera. He had previously isolated the bacteria and prior to taking a holiday he left specific instructions for his assistant, Charles Chamberlain (our hero by chance), to infect the chickens with the latest cholera culture. However, Charles forgot to administer the bacteria and left for vacation instead.
Upon his return he exposed the chickens to the now month old bacteria. The chickens took a slight illness, but soon were healthy again. After re-exposing the healthy chickens to new (and full-strength) cholera bacteria he discovered that they were completely resistant. Vaccination had been discovered. One of the most important discoveries in recent human history was basically the result of a mistake. Vaccination was discovered because Charles Chamberlain was in a hurry to get out of the lab and on a holiday.
On the other end of the spectrum is blowback. Blowback as a term originated in Military Intelligence circles to refer to the negative un-intended consequences that arise from clandestine operations.
Perhaps the most relevant current example of blowback in today's era is Osama Bin Laden. Everyone is well aware of the US involvement in funding anti-soviet forces in 1980's Afghanistan and many have noted that Osama was a US creation, but it's not really that simple.
Originally from Saudie Arabia, Osama Bin Laden left college and joined the afghan independence fight in 1979. By 1984 the independantly wealthy Bin Laden was in charge of an anti-soviet group called MAK (Maktab al-Khidamar); MAK was a primary recipient of support from Pakistani Intelligence Service (PIS); which in turn was a favored agency through which the CIA funneled money into the afghan resistance. With CIA assistance the PIS is estimated to have trained 80,000+ Afghan mujahideen for anti-soviet operations.
But, despite the billions of dollars that the US funneled through PIS into groups like MAK -- it was an estabilished rule that US officers should avoid direct contact with the mujahideen and that they would not have direct oversight into how the money would be distributed.
Thus the US made a calculated decision to outsource it's management of funding of the anti-soviet afghan insurgency to the Pakistani Intelligence Service. Obviously the Pakistani Intelligence Service had very little motivation to ensure that the money was distributed to those whose long term goals aligned with US interest. Thus bin laden rise to prominence was a result of US funding and pakistani decision making in allocating those funds.
Through his leadership in MAK; Osama Bin Laden built a personal network and by 1988 he had split off from MAK to form a islamic military operations that is group now known as Al Qaeda. Without the support of the US and PIS it's entirely possible that Al Qaeda would've never been created.
Every decision has unintended consequences and those effects can alter the course of history. What decisions are you currently mulling over? Are there consequences beyond what you imagined? Can you even prepare for something that's not even on your radar screen?