Sorry to thread necro, but yes even a stopper can flavor a batch, especially with just 1 gallon jugs. I just did some different ciders in 1 gal jugs and a couple stank like rubber. I didn't feel like sanitizing the thief so I just poured samples... right across the glass the stoppers were in contact with. They tasted like fresh asphalt. The good batches has silicone stoppers. The noxious ones had stoppers that I ran out to a local brewing section of a grocery store for. The entire siphoned jugs tasted off too, not just the samples I poured. It leeched across the headspace right into my cider.
I looked at the brand on them. "Fermenthaus".
I tried boiling them, and the smell just got worse. It gives me headaches. And on the top of the water and all over the pan was a sticky, oily film of rubber stank. Just everywhere. The stoppers are getting thrown out and the cider given to some less picky friends. Never again with these Fermenthaus stoppers. Silicone only for me now.