Recent progress in the development of table-top sources of single-cycle terahertz (THz) pulses has opened up new opportunities for experiments on ultrafast control of spontaneous polarization in ferroics [1]. Nearly single cycle intense THz pulses induce in Ba0.8Sr0.2TiO3/MgO generation of transient second harmonic from near-infrared pulse. The dynamics of the nonlinear optical signal is characterized by a step and oscillations at the frequency of about 1.67 THz. The step and oscillations can be mistakenly interpreted as a switching of ferroelectric polarization in Ba0.8Sr0.2TiO3 and coherent soft mode excitation, respectively. Here we demonstrate that the dynamics of the non-linear signal must be explained in terms of interference of static and relativistically moving sources of the second harmonic in the studied structure. Importance of propagation effects in interpretation of results of nonlinear optical time-resolved experiments cannot be ignored. Although Second Harmonic Generation is established as a powerful probe of ferroelectricity [2, 3], time-resolved measurements of ultrafast dynamics in ferroics with the help of this effect can be hampered by the presence of several interfering sources of the detected radiation. In particular, the dynamics triggered by nearly single cycle THz pulse in BST/MgO structure can be mistakenly assigned to the dynamics of ferroelectric order parameter. Our experiments demonstrate that the observed non-linear signal must be explained in terms of interference of static and relativistically moving sources of the second harmonic in the studied structure.