If you have a RootClass with a SubClass as a child property and it has two float properties then BsonSerializer.Deserialize<TheClass>(json); will throw an error saying that it cannot deserialize the second float. See this gist for re-pro code:

https://gist.github.com/Ravenheart/0f4f47bbe14737cb01998cf86bf0396d

This error occurs in ALL versions of the C# driver, including the legacy one.