Working too late in the shop and literally forgetting that locking pliers exist.
I was using some pliers to hold something together with one hand, and holding a bolt in while simultaneously trying to thread the nut on with the other. I actually got up to find a small C clamp and looked in my box. Upon seeing the locking pliers, I knew it was time to put up the tools and call it a day (night).
The project is done though and that was the point, so I wouldn't have to dick with it tomorrow.
The story of three wheels and a man...