Oxymercuration demercuration, PCC, NaCN/HCl reaction
Check the mechanism of the last reaction here. 
http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch20/ch20-3-5-1.html

It is not the displacement of nitrile by carboxylic acid(which you do not have, btw). It is the combination of water addition and the displacement of ammonia by second equivalent of water.