Filling of $3 d$ orbital starts after the filling of $4 s$ orbitals. If the Aufbau principle had not been followed, the filling of $3 d$ orbital would have been prior to filling of $4 s$ orbital and the electronic configuration of $Ca$ would have been $1 s ^2\, 2 s ^2 \,2 p ^6\, 3 s ^2\, 3 p ^6 \,3 d ^2$ and it would have been placed in the $d$-block.