Have refrigerator, empty, at priority 5
Have critter feeder, full, at priority 6
Make Gristle Berry (or any food that could go in either one)
Dupe will endlessly pick up the Gristle Berry, drop it, pick it up again, drop it, forever
Disallow Gristle Berry from Critter Feeder to break the endless loop
Before they pick it up, they seem to know it can go in the refrigerator (else why would they pick it up). But then once they've picked it up, they suddenly decide it can't go anywhere, likely wanting to put it in the Critter Feeder and not looking at the next option when it is full.