Yup. Your mind can only process 24 frames a second. That's why movies are 24 frames a second. Literally your bodies are completely unable to process the information fast enough. And as was mentioned, it can happen at any time.

The slendermen are very professional and quick when they are given the go a head. Most of the targets on the list don't even feel the nail slide in their skull and pierce their pineal gland or the disconnect of the corpus callosum that holds the hemispheres of the brain together. The rest of the nails in the cart are just cause they deserve it. Most of them don't deserve an open casket so they help remodel the window if they are high enough up and they make a big mess.