## Trying to Find a Counter Example

I'm having a hard time with this problem, could someone point me in the right direction?

How can I show the following is false?

For all sets $A$ and $B$, and all functions $f : A \rightarrow B$, $g : B \rightarrow A$, if $g \circ f = I_{A}$, then $f \circ g = I_{B}$.

Obviously, I need to find a counterexample, but I'm having a hard time coming up with one.