Expert witnesses are a valuable tool. They are not, however, always perfect, prepared to give the testimony you had envisioned, or able to give testimony that is helpful to your case. So, what is the possible fallout when you reach that critical moment where you decide you cannot or should not call an expert you have named as a witness or—even deeper in—have referred to the expert in opening arguments as the one with testimony that will convince the jury the other side's expert is wrong? Depending on your jurisdiction, you should think twice about not calling the expert witness to avoid the risk that the jury will be given a missing-witness, adverse inference instruction.
The colloquial "missing-witness" rule—which developed from a century-old U.S. Supreme Court decision, Graves v. United States, 150 U.S. 118 (1893)—allows one party to obtain an adverse inference against the other for failure to call a controlled witness with material information. The rule, put in place well before the adoption of the Federal Rules of Evidence and the Federal Rules of Civil Procedure, is alive and well in both federal and state courts.
Application of the rule, however, is anything but uniform. Both among the federal circuit courts and the state courts, the rule is sometimes never applied, sometimes applied only to fact witnesses, and sometimes applied to experts just as equally as it is to any other witness. In addition to variation across courts, even within circuits and states, application is varied. If you think your expert may end up getting shelved, it will be important to research the issue in your jurisdiction and know the scope of the risk associated with failing to call the expert.