I saw the question( When is an almost geometric quotient flat?) that which said "The quotient $\pi$ is flat if and only if $\pi$ is equidimensional and $X$ is smooth".
"The quotient $\pi$ is flat if and only if $\pi$ is equidimensional and $X$ is smooth".
I am curious is there an example that $\pi$ is not equidimensional. Also where can I see the original reference for the statement?