Inspite of long range order in the arrangement of particles why are the crystals usually not perfect?

Crystalline solids or crystals have long range order, that is, they have repeating pattern of identical unit cells over a long distance over the entire crystal. Despite this property, crystals are usually not perfect because of deviations or “defects”. The crystalline solids usually consist of a large number of small crystals, each having a definite shape. So, defects in the entire structure may either happen because constituent particles may not get the time to arrange themselves in the correct order during formation or inclusion of impurities in the crystal lattice. The imperfections caused by impurities in lattice are of two types; point defects which occur due to deviation from arrangement of an atom and line defects which occur due to deviation from arrangement of rows of atoms in structure.