Unless there are any legal precedents of relevance to the contrary, my own view (IANAL) is
that the idea, that someone can sue someone else who has placed source code into the public
domain for damage caused by it, is FUD without practical foundation. Anyone using public
domain software source code for a potentially damaging purpose could reasonably be expected to
have what it does examined by an expert in order to confirm its suitability before using it
for such an application. You might as well try to sue someone else for a published idea or
research which you misapplied and which went wrong when you did so; this course of action
would also not get the litigant anywhere in the courts.
I can imagine an exception if source code for a trojan was placed in the public domain,
particularly if the source features making this program a trojan were obscure, and good
evidence existed that the author intentionally and/or maliciously included these hidden and
potentially damaging features. But I don't think applying a free software license to such code
would protect the author of it from similar litigation under these circumstances either, as
any disclaimers in this license would be considered moot.
Personally I don't think spreading the FUD: that releasing well-intentioned software into the
public domain can make the author liable - will attract any programmer or decision-maker to
apply free software licenses to code who otherwise wouldn't, though it might turn some people
off free software altogether.