Imagine this scenario:
Red has a reach weapon and he's threatening B1 and B2.
Green has a normal weapon and wants to get closer to Red in order to attack him. ( C2 )
So green takes a first step into the reach of Red's weapon, but provokes no attack of opportunity (not yet).
Green then takes a second step into C2, leaves the threatened area of red and arrives in C2.
This should normally provoke an attack of opportunity, but since Red is not threatening C1 and C2, I'm not sure if he gets the AoO.
Green is using no particular feat or acrobatics check to avoid the AoO.
Actually, I found no evidence in rules as written, so any reference is gladly welcome.