For what real values of $a$ is the expression $\frac{a+3}{a^2-4}$ undefined? List your answers in increasing order separated by commas.
When the denominator is 0, the expression is undefined. Therefore, we set the denominator to 0 and solve: $$a^2-4=(a-2)(a+2)=0.$$ Therefore, the expression is undefined when $a=\boxed{-2, 2}.$