In situ trace gas (O3, NO, NO2, NOy) and ultrafine aerosol particle data from a passenger aircraft (project CARIBIC) and the NOAA WP-3D research aircraft are used to identify the origin of ozone-rich transients that occasionally appear in aircraft ozone data sets along flight distances of 5--80 km. Evidence of ozone import from the stratosphere, once suggested as the most likely cause, cannot be found. Our data rather reveal that the majority of the recorded ozone transients are artifacts caused by electrical discharges on the aircraft fuselage and the sample air inlet system. These discharges produce not solely O3 but also nitric oxide (NO) which rapidly reacts with O3 to form nitrogen dioxide (NO2). Further evidence is, however, provided showing that some of the less pronounced and broader O3-NO-NO2-rich transients are of atmospheric origin. We hypothesize that they are formed in an early (i.e., prelightning) phase of thunderstorms due to cold electrical discharges on the surface of charged hydrometeors. Simple considerations suggest that the amount of O3 and NO produced through these mechanisms is negligible with regard to the global tropospheric budget for the two gases.