RuBP oxygenase-carboxylase (rubisco), a key enzyme in photosynthesis, is the molecular equivalent of a good friend with a bad habit. In the process of carbon fixation, rubisco incorporates carbon dioxide (\text{CO}_2CO2) into an organic molecule during the first stage of the Calvin cycle. Rubisco is so important to plants that it makes up 30\%%30%% or more of the soluble protein in a typical plant leaf^11. But rubisco also has a major flaw: instead of always using \text{CO}_2CO2 as a substrate, it sometimes picks up \text O_2O2 instead.
This side reaction initiates a pathway called photorespiration, which, rather than fixing carbon, actually leads to the loss of already-fixed carbon as \text{CO}_2CO2. Photorespiration wastes energy and decreases sugar synthesis, so when rubisco initiates this pathway, it's committing a serious molecular faux pas.