If Aufbau rule is not followed, K – 19 will be placed in the block

Since K has atomic number 19,

$1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^1$ 

The final electron will enter the d-orbital. This makes it a part of the d-block elements.