Say there would be plants that release a lot of N2O (nitrous oxide or laughing gas). On Earth, N2O is a trace-gas and it is pretty stable. In our atmosphere it has a half-life of > 100 years, and as I understand it, it is destroyed in the stratosphere by ultraviolet with wavelengths of < 230 nm.
Doing a back of the envelope calculation, there would be a strong build-up of N2O in the atmosphere if plants would release half, or even a tenth as much N2O as they release oxygen.
My question: what would happen if there was 10 or even 100 mbar of N2O in an Earth-like atmosphere? More specifically: 2N2O => 2N2 + O2 is an exothermic reaction. I can imagine that at a given threshold the reaction could become self sustaining so that there exists a strict maximum partial pressure of N2O.