Louis Braille, the inventor of the tactile Braille alphabet, went blind after he—as a small child—was attempting to poke a hole in a piece of leather with an awl, slipped, and stabbed himself in the eye. Subsequent to this, infection set in and he lost sight in both eyes.

The Braille Heuristic is that whenever you find yourself in a tool/workpiece/face geometry similar to what Braille was in before he stabbed himself in the eye, you should immediately stop what you're doing.

I credit having not yet stabbed myself in the eye with knowledge and application of this heuristic.

I know that many of the people who will read this are highly skilled professionals, all I'm saying is: just think about it.