Can an employer dismiss an employee for a one-off act of poor performance?
Employers should not normally dismiss an employee for a one-off act of poor performance. In most cases, for a poor performance dismissal to be reasonable, the employer will need to have issued warnings previously, particularly as incapability is usually assessed over a period. The Acas code of practice on disciplinary and grievance procedures recommends that employers give at least two warnings before they dismiss for poor performance.
Dismissing an employee without warnings is unlikely to create a problem for the employer if the employee does not have the required two years' service to bring an unfair dismissal claim, and has no basis for claiming that the dismissal amounts to unlawful discrimination or is for one of the automatically unfair reasons for dismissal for which there is no service requirement. However, it would be good practice for the employer to provide an underperforming new recruit with the training and support necessary to enable them to improve, before taking the decision to dismiss.
It may be lawful to dismiss for a one-off act of poor performance if it constitutes a very serious case of negligence endangering life or limb. In Taylor v Alidair Ltd  IRLR 82 CA, the Court of Appeal held that it was fair to dismiss an airline pilot whose negligent landing put the lives of passengers at risk. The employee's actions amounted to gross incompetence and could have led to calamitous consequences. The airline was entitled not to take the risk of a recurrence.