Simple answer, lots of stuff. Also on a warm sunny day just about any insect that flies will find your fresh gloss coat irresistable. But main concerns tend to be temperature, its often not as warm as you think, moisture, that one rain drop from a cloudless sky will land on your model, and wind - even a tiny breeze can be a pain.
Also you have to move your freshly painted model indoors immediately or all of the above will make their mark in the fifteen minutes or so the paint is drying.
So, not ideal, but sometimes you have no choice.
Oh, and don't spray resting on the patio table, side wall or anything else without covering it. Unlike the workbench, you will be nagged about overspray.
Matt