Stoichiometry and POAC Contradiction Lets say we have the reaction 

$$\ce{H2 + O2 -&gt; H2O}$$

And we are given 10 moles of $\ce{O2}$ and are asked that 
How many moles of hydrogen gas do we need to complete the reaction ? 

Stoichiometry gives the answer as 10 moles but by applying Principle Of Atom Conservation,  we get 

$$\text{Moles of }\ce{H2}  =2\cdot\text{moles of }\ce{O2}$$

Why is the POAC giving wrong results?
You applied the POAC incorrectly . Even your stoichiometric answer is incorrect.

First Balance the reaction 

$$\ce{2H2 + O2    -&gt; 2H2O}$$

We get that one mole of $\ce{O2}$ requires 2 moles of $\ce{H2}$.
Thus 10 moles $\ce{O2}$ need 20 moles of $\ce{H2}$.