When you're running at least two extension cords to power something, a rough tug on the line can easily separate them. A carefully placed carabiner fixes that.
In the video above (at about the 3:08 mark), YouTuber CrazyRussianHacker, demonstrates the perfect placement for a carabiner if you don't want your cords coming unplugged. Just plug the extension cords together, hook both cords inside the carabiner, and pull. The carabiner will only slide so far down the cords and keep them unplugging. Of course, if you don't have a carabiner on hand, you can always knot them together in a pinch.
7 Carabiner Life Hacks [YouTube]