It seems obvious, but some employers often forget that the punishment must fit the crime when it comes to issues of conduct. The true legal test is whether the decision was within the band of reasonable responses for the employer to take.
I was speaking to someone today who had an employee caught stealing and who admitted the offence. They were advised (and not by us, I hasten to add) to issue a warning. Very safe advice but theft is Gross Misconduct and could warrant dismissal or a final written warning. The other extreme is where we acted for an employee who was dismissed for Gross Misconduct when one of her direct reports told her wrongly that something had been done. The punishment did not fit the crime in that case.
When carrying out training for employer clients, one exercise we sometimes do with management is getting them to distinguish between red and yellow card offences, to use a football term. It is suprising how lenient some are and how extreme others are. If in doubt seek advice, otherwise you could be left with a problem employee you do not want or be facing a Tribunal claim!