Employer can only dismiss an employee if there has been gross misconduct. Gross misconduct is serious enough to dismiss on the first offence, whereas misconduct is likely to involve giving the employee a second chance after giving formal warning.
It is strongly advisable to give employees a clear indication of the type of behaviour employer consider to be gross misconduct. Employer can do so in the contract of employment itself or in a staff handbook. Identifying such behaviour in advance will help to demonstrate later on that you regard it as significant.
Including the incident of such mistakes, in the employment contract and staff handbook ,as gross misconduct which would result in dismissal.
If the matter goes to Employment tribunal , court decides following factors to determine if the dismissal was fair or not-
Did Employer have a genuine belief in the employee’s guilt?
How thoroughly employer investigate the alleged offence?
Was the employee given all of the information that had been gathered as part of the investigation?
Did employer give the employee an opportunity to state their case?
Were they entitled to be accompanied by a work colleague or trade union representative?
Did employer hold a disciplinary hearing, chaired by an impartial individual?
Did employer give them time to prepare for the disciplinary hearing?
Was the offence gross misconduct, as set out in your disciplinary procedures, and was the employee aware of the penalty that could be imposed as a result of that misconduct?
Were there mitigating circumstances or other facts that should have been taken into account, for instance, health or domestic problems, provocation or ignorance?
Were alternatives to dismissal considered?
If the above factors were not taken into account by your employer then you have a claim for unfair dismissal.