Very unlikely this would occur. Jury deliberations are considered sacrosanct in American jurence prudence. You are under no obligation to explain your reasoning to anyone when deciding a verdict. And courts have been traditionally very wary of allowing prosecutors to delve too deeply into them to go on fishing expeditions unless a very specific procedure has been shown to have been violated.
The prosecution would essentially have to read your mind to demonstrate that you somehow "lied" in answering the questions during jury selection, which is why what you're describing virtually never happens. And there's always the plausible defense that you simply changed your mind as the trial unfolded.
This sounds reasonable - but doesn't refute my main point - that nullification, while not a criminal act, can often not be applied without first committing a separate criminal act, and that one should be aware of this fact and take appropriate precautions.
