  Harmonics are quite common in pulsating stars, and they are always considered
to mimic the behaviors of their independent parent pulsation modes and not
taken as the key information for asteroseismology. Here, we report an SX
Phoenicis star XX Cygni, whose periodogram is dominated by the fundamental
frequency $f_{0} = 7.41481 \pm 0.00004\ \mathrm{cd}^{-1}$ and its 19 harmonics.
According to the analysis of the archival data from \TESS, we find that both
the amplitudes and frequencies of the fundamental mode and the harmonics vary
within TESS Sectors 14-17 and 54-57, which might be caused by the contamination
by neighbouring stars. What is more interesting is that, the harmonics show
significantly uncorrelated amplitude and frequency variations over time. Some
possible origins and interesting issues are proposed to scheme the further
research of this hidden corner in current asteroseismology.
