A release of nitrogen oxides (NOx) at a chemical plant resulted in a reddish cloud that hovered over the site before dissipating due to the presence of strong winds. This NOx discharge was caused by an erroneous value indicated by a flow meter placed on the nitric acid inlet line of a reactor at a glyoxylic acid manufacturing unit. Other simultaneous malfunctions were also observed. Following a programming error, the low-level alarm bypass triggered the high-level alarm bypass as well, with the flow rate exceeding the flow meter measurement range. Moreover, the control room operator had not noticed 3 or 4 alarms tripped during the 20 min he spent controlling installations. These multiple operating breakdowns caused a valve on the production line to open when it should have remained closed, as the quantity of nitric acid in the reactor at this point of the reaction was sufficient. Given the exothermic nature of this reaction, reactor temperature rose to a level that triggered dilution of the reaction mixture in water and a safety shutdown of the reactor, i.e. its drainage into an empty atmospheric pressure vessel designed for this purpose followed by degassing of the vessel. No injuries were reported. All plant flow meters were subsequently inspected.