Angus Scrimm, better known as The Tall Man in the Phantasm films who has an appearance similar to The Operator/Slenderman has a very brief cameo as a patient in a psychiatric hospital.
Production for the film was brought to a halt after two instances of youth violence relating to the "slender man" myth appeared in headlines.
When the group stops for gas, a missing poster can be seen for Alex Kralie, one of the main characters of the Marble Hornets web series.
The creature in the story is never referred to as the "Slender Man," though the creator of the web series admits it was inspired by the legend.
The first theatrically released film for Marble Hornets which is a YouTube web series.