What common mistake can lead to paint runs or sags?

Applying excessive amounts of paint in a single layer can indeed lead to paint runs or sags. This occurs because when a thick layer of paint is applied, the weight of the paint can overcome its own viscosity and flow downwards, creating runs. Paint is designed to be applied in thin, even coats to ensure proper drying and adhesion. When too much is applied at once, it doesn't have the opportunity to dry evenly, leading to an unsightly finish.

In contrast, the other options, while they can lead to various painting issues, do not directly cause runs or sags. For instance, painting on a cold surface could lead to poor adhesion or slower drying times, but it wouldn't inherently create the same kind of dripping effect caused by over-application. Using the wrong type of paint can affect color, gloss, and durability, but again, it is not a direct cause of sagging. Lastly, not priming a surface can lead to other adherence issues or uneven paint absorption, but it wouldn't generally result in the runs that occur from too much material being applied at once.