Matt E. , loose terminal or a poor connection between the contacts of the outlet & the prongs on the plug... The outlet didn't appear that old, (Leviton's low end spec grade, N5-20) It is connected to stranded wire on the screw terminals... maybe not enough surface contact for the load (commercial freezer)

Scott, the snack bar is in Huntington State Beach lot... (@ Magnolia St)If I remember right, these things cant be more than about 10-15 years old...
I never would've considered EMT out here, all of 200-300' from the ocean itself! Notice in the pic of the crumb ling EMT, it was the EGC also! [Linked Image] The switch connected to it is missing its WP cover also. The bypass lug might've been tightened & the corrosion (which doesnt show very well in the pic) may have something to do with it's failure..

-Randy