Invasive aquatic plants
When we started looking for pond plants, they suggested a number of plants that I later learned weren't native to North America. Moreover, parrot feather (Myriophyllum aquaticum), is potentially invasive to waterways. Although my pond doesn't connect to a waterway, parrot feather will grow in soil as well, and so it could eventually move out of my yard.
Another non-native plant they sold me was yellow flag iris (Iris pseudacorus). It's attractive, but some states are reporting that this plant has become a nuisance.
For a while, we also "planted" water hyacinth (Eichhornia crassipes), an interesting plant with an air bladder. It currently cannot overwinter here in Central New York, but in a future with climate change, who knows? I doubt it could be harmful at the moment, but I think it's important to establish viable landscapes now instead of waiting until it's an emergency.
We removed all of these plants from our pond.
LESSON: I learned to research first and buy later. I now know that garden centers will sell anything people will buy!