Copper is known to be versatile in producing various products from electrochemical CO2 reduction reaction (eCO2RR), and the product preference depends on reaction environments. The literature has reported that alkaline electrolytes favor acetate production and proposed hypotheses on reaction pathways accordingly. However, our work shows acetate can also come from the fast non-Faradaic chemical oxidation of acetaldehyde in alkaline environments. This adds uncertainties into measurements of both acetaldehyde and acetate production and leads to untrustful investigations on the reaction mechanism as a consequence. With an electrochemistry-mass spectrometry combined (EC-MS) system, we not only demonstrate why and how the imprecise acetaldehyde and acetate production occurs in previous research but also present immediate detection of acetaldehyde as a function of applied potential on single crystal Cu electrodes during electrochemical CO reduction reaction (eCORR).