Old law, Otis Engineering Corp. v. Clark, 668 S.W.2d 307, 310 (Tex. 1983):

Even though courts have been reluctant to hold an employer liable for the off-duty torts of an employee, “[a]s between an entirely innocent plaintiff and a defendant who admittedly has departed from the social standard of conduct, if only toward one individual, who should bear the loss?” W. Prosser, supra, at 257. Dean Prosser additionally observed that “[t]here is nothing sacred about ‘duty,’ which is nothing more than a word, and a very indefinite one, with which we state our conclusion.” Id.

At 311:

Therefore, the standard of duty that we now adopt for this and all other cases currently in the judicial process, is: when, because of an employee's incapacity, an employer exercises control over the employee, the employer has a duty to take such action as a reasonably prudent employer under the same or similar circumstances would take to prevent the employee from causing an unreasonable risk of harm to others.

New law:  Nabors Drilling, U.S.A, Inc. v. Escoto, No. 06-0890, 52 Tex. S. Ct. J. 885 (Tex. 2009):

We hold that because Nabors took no affirmative action as a result of any perceived employee fatigue or incapacity, as required by Otis, Nabors owed no legal duty to the plaintiff in this case.

Rule:  if you send a drunk home early, you are liable, if you let an exhausted employee drive home after shift (and keep your supervisor's mouth shut), you are not liable.

Rule:  Never, never, never comment on a drunk on the job, or one who is leaving the job.  Watch him or her drive off the property without comment and you are off the hook.  Then, under Nabors, you escape the duty of Otis Engineering.