Radio transmission and nuclear energy would be my first ideas for technologies which almost no-one would assume can exist without a modern physics education or an existing proof of concept. It's hard to come up with earlier stuff than that. Hot-air balloons were probably very surprising, but the principle isn't that hard to grasp in retrospect. Electricity falls outside naive physics, but you can still observe lightning in nature. Nukes and radio on the other hand just plain shouldn't exist if you asked anyone well-educated and level-headed circa 1800.