Small change, big difference

Solution 1:

I'd say pivotal fits the bill nicely.

http://www.macmillandictionary.com/dictionary/british/pivotal

Extremely important and affecting how something develops.

• A pivotal moment in the history of the Egyptian state
• He played a pivotal role in the negotiations.
• A pivotal figure/player in US politics
• Pivotal to: He sees these reforms as pivotal to our economic interests in Asia.

From http://dictionary.reference.com/browse/pivot

A pin, point, or short shaft on the end of which something rests and turns, or upon and about which something rotates or oscillates.

Solution 2:

This is nothing but the butterfly effect.