In Iain M. Banks' Surface Detail there's a scene where a piece of gold is dropped into some mercury and eventually bobs back to the surface. It's particularly mentioned it's still "a shining ingot."
Except that if you tried this for real the gold would instantly be coated with a layer of dull grey gold-mercury amalgam, the stuff that fillings used to be made of. I did this accidentally once with a gold watch, just a few seconds in contact with drops of mercury was enough.
Just another example of why it's always a good idea to get your hard SF read through by someone who knows some science...
And I just failed my own test! Someone just pointed out that the gold wouldn't float anyway!