I just tried it with a barrel and it worked correctly. I had never used a trigger zone before so really wasn't sure what I was doing, but I set the Barrel (Acid) 's properties to Spawn at Start = no, Spawn after delay = yes and number to spawn to 1. In the trigger zone, If Used = Barrel (Acid). Hope that helps.
EDIT: I just changed Spawn after delay to no and number to spawn to 0, and it still worked.
MAME Cab PC: i7 4770@3.4Ghz (passmark 9945), 12GB RAM, Win 8.1/64, GeForce GTX645 (passmark 1898); Shiny new laptop: i7 4800MQ@2.7Ghz (passmark 8586), 16GB RAM, Win 8.1/64, GeForce GTX870M (passmark 3598); Old laptop: i5@2.3Ghz, 8GB RAM, Win 7/64, Intel 3000 graphics