This could be the scariest prank ever, if I had someone to play it on:

Step 1. Trick the victim into graphing this slightly modified version of Tupper’s Self-Referential Formula:

$\displaystyle z= \left\lfloor \mathrm{mod} \left( \left\lfloor{\frac{y}{7}}\right\rfloor 2^{-7\lfloor x \rfloor - \mathrm{mod}(\lfloor y \rfloor, 7)}, 2 \right) \right\rfloor$

in the region, say $-10 \leq x \leq 150$, $n \leq y \leq n+8$, where n=

381025769433284440392319964900539302335
336506037124716737876327885349255244053
654529126222910795091774837392244419620
805572082941110711057252345724567742835
679828220923525384477934725633019757953
249733463908820850208792361145516746285
337177182564212162731442.

Step 2. Existential terror.

(Graphing the formula in that rectangle gives you the message “help! I’m trapped in a Universe factory”, which of course is a reference to this classic xkcd)