Leave the Auditorium for the next 90 seconds.

Rumor has it that Herschell Gordon Lewis ordered a pint of stage blood for his upcoming movie, but ended up with a barrel of the stuff instead and decided to make 'Blood Feast', incidentally inventing the splatter genre. True or not, it makes for a good story. Critics were relentless and called the movie "an insult even to the most puerile and salacious of audiences".
For today's standards, the special effects are very poorly executed, nontheless the movie has an eerie and unsettling quality to it. Watch the trailer at your own peril (NSFW).