I once flew with another pilot in a Champion Challenger (the fore-runner of the Citabria). No headphones (no radio either), but it had retro-fitted shoulder harnesses for the front-seat.
While taxying for takeoff, my passenger pulls on my shoulder harness. I turned around and he gestured at the shoulder harness with a quizzical look on his face. I shook my head.
After takeoff, there is the previously described "nasty rhythmic banging sound". Opened the door and my passenger pulled in the lap strap and buckled it up. I had misinterpreted his query. He thought the shake of my head meant no shoulder harness and no lap strap.
A good job it wasn't a rough day!
The moral of the story - do a proper take-off briefing and harness checks.