Two factors complicating thermal expansion of crystals are given. (1) Heating of a metastable crystal phase at low temperatures represents usually thermal expansion only. At higher temperatures, above a glass transition temperature T_g, structural reconstruction to a stable phase is involved into the expansion. (2) The atomic vibration size in a direction of the longest vibration amplitudes can be sharply increased. In this case on heating above definite temperature, neighboring cations that were not in touch start to repel each other along this direction - an additional mechanism of thermal deformations switches on. The both these phenomena show as special points on temperature dependence for unit cell parameters.