About a week ago in a Citabria, I had a student mistake the trim for the throttle. The knobs are exactly the same part, the trim is located just below an forward of your left elbow. The throttle is just above and forward... No big deal, it's happened before. Student goes to reduce power on the downwind and all of a sudden you're slammed back in your seat and the nose is pointed to the sky (you can practically fly a loop with the trim alone).
Last week was different. He didn't apply enough wind correction on base and we got pushed out a little farther our from the runway, this made us a little low on the approach as well. Our TPA only sits at 600AGL so there's not much room for error. After turning final I mentioned to him that it might be a good idea to add a little power so we could make it to the runway... Then the nose started diving toward the ground. I grabbed the stick, yelled. "my airplane", and started pulling and the pressure was fighting me and getting stronger (I thought it was him still pushing). Then I realized I never heard any power being added, looked down and say his had on the trim running it forward, pushing the nose down, thinking he was adding power! The lower we got the more "power" he was trying to add! We recovered on final well below 100 ft. If anyone had been watching I can't imagine what they would have thought...
Even after 3300 hours of dual with about half of that spent spinning and/or upside-down, something new will manage to pop up now and then and help add to the gray hair collection. It's ok to be relaxed, just don't get complacent.