Expert witnesses can be sued only under very specific circumstances. Traditionally, witnesses brought before a court are immune from liability, even if they make a mistake in their testimony. This is to prevent witnesses from being discouraged from testifying by the threat of a lawsuit from the opposing party. That said, expert witnesses who knowingly present false information under oath are subject to charges of perjury, just like any other witness testifying in court.
Additionally, recent court rulings have allowed suits to be brought against expert witnesses who are negligent or fraudulent in their professional duties. A lawsuit against an expert witness is typically undertaken only when that witness commits a provable act of malpractice or other fraud. Indeed, some law firms have sued their own expert witnesses for breach of contract after discovering that the witnesses were lying or presenting fraudulent evidence to the court. Expert witnesses cannot be sued simply for giving evidence or forming an expert opinion that the opposing party disagrees with. The expert opinion might be challenged in cross-examination, however, or be rebutted with evidence or additional expert witness testimony.