This is not a case of mutual mistake (Sec. 152), or unilateral mistake (Sec. 153) making a contract voidable under the Second Restatement of Contracts:
152. When Mistake of Both Parties Make a Contract Voidable
Where a mistake of both parties at the time of contract was made as to a basic assumption on which the contract was made has a material effect on the agreed exchange of performances, the contract is voidable by the adversely affected party unless he bears the risk of the mistake under the rule stated in 154.
...
153. When Mistake of One Party Makes a Contract Voidable
Where a mistake of one party at the time a contract was made as to a basic assumption on which he made the contract has a material effect on the agreed exchange of performances that is adverse to him, the contract is voidable by him if he does not bear the risk of the mistake under the rule stated in 154, and
the effect of the mistake is such that enforcement of the contract would be unconscionable, or
the other party had reason to know of the mistake or his fault caused the mistake.
Dr. Bob knew what he was trying to get so there was no "mutual" mistake under Sec. 152.
Also, there is no reason to think that enforcement would be unconscionable or that Dr. Bob knew of or caused any alleged mistake, so there is no out for the seller under Sec. 153.
That said, in most real world circumstances the existence of a contract means squat
