  Akin to zero-mode anomalies, such as the chiral anomaly of edge states in
quantum Hall effect, in this work, a pi-mode anomaly is proposed in a 1+1
dimensional periodically-driven topological/normal insulator (TI/NI)
heterostructure. Usually, when coupling in a background gauge field, the zero
modes on domain walls would provide an anomalous current term that is
eventually canceled by additional boundary contributions from the topological
bulk, via the Callan-Harvey mechanism. This anomaly cancellation associated
with the generalization of bulk-boundary correspondence is called anomaly
inflow. Through our photonic modeling and setup of the Floquet TI/NI
heterostructure, for the first time, we experimentally observed the $\pi$-mode
domain wall in certain driven frequencies, which is always attached to the
reminiscent Floquet gauge that plays the vital role of an emergent background
field. Indeed, due to the possible emergence of Floquet gauge anomaly from the
driven topological bulk, the resultant $\pi$-mode anomaly can be matched on the
driven interface between Floquet domains. Prospectively, we believe our
prediction and observation could pave a new avenue on exploring anomalies in
both periodically-driven classical and quantum systems.
