Operando spectroscopy is critical for the understanding of electrocatalytic reactions. The structural dynamics of NiMo, one of the most active non-precious catalysts for the H2 evolution reaction (HER) under alkaline conditions, were studied by X-ray absorption spectroscopy (XAS) and Raman spectroscopy. We observed considerably unexpected results, some of which may change the way we look at data interpretation. Under operando HER conditions under XAS or visible-light laser irradiation, trace amounts of Mo ions were liberated and then redeposited on the electrodes as a Mo4+-containing oxide, giving the appearance that the HER-active catalyst is in the oxide phase. The Mo oxide self-propagated under sufficiently energetic light, resulting in a pattern matching the irradiated catalyst area. Detailed spectroscopic characterization of the role of Mo in NiMo may be hindered under operando conditions and care needs to be taken to avoid misleading data interpretation.