It used to be campfires that people could exploit for fun and profit, but we stomped out their ability to be stacked to the moon a good while ago. Furnaces are basically big campfires, so it’s only expected that they have lots of exploits for people to take advantage of. And being Rust, they’ve done so in spectacular fashion.
Rust forum user
Raidenz spent four hours making a house of furnaces, building what he considered to be an unraidable shell around his base. Though the piles of protective smelters aren’t the prettiest, I can’t help but wish I’d seen them burning in the night. Anyway, Raidenz’s unraidable Base was raided in the time it took him to upload this video.
On the official Seattle server, milkman2040 discovered that you could
stack furnaces and use them as a staircase to the top of the server? Why? Because it’s there, that’s why. Their line of hot, sexy stone could be seen for miles.

Where are people getting all these furnaces from?