Eating food that has been left out unrefrigerated for an extended period can pose risks of foodborne illness due to bacterial growth.
In the case of pesto or any other perishable food item, the general rule is the "2-hour rule." Perishable foods should not be left at room temperature (between 40°F and 140°F or 4°C to 60°C) for more than two hours. This time frame is reduced to one hour if the ambient temperature is above 90°F or 32°C.
If the pesto has been left unrefrigerated overnight, it has likely been at room temperature for far longer than the recommended safe window. As a result, it's better to err on the side of caution and discard it rather than risking potential foodborne illness. Bacteria can grow rapidly in certain conditions, and consuming contaminated food can lead to symptoms like nausea, vomiting, diarrhea, abdominal pain, and more severe illnesses in some cases.
Remember, when in doubt, throw it out. To prevent food safety issues in the future, always follow storage guidelines and keep perishable foods refrigerated when they are not being actively consumed.