Employers and employees alike should exercise a small measure of caution at work functions.
Employees: Be sure not to cause "embarrassment" to your employer! Unruly, offensive behaviour at a work function attended by people outside your organisation could lead to your justified dismissal if your actions caused embarrassment and disrepute to your employer.
Employers: Be careful about your venue, if may be better to have your party offsite. For instance it may not be advisable to have alcohol consumed in a workplace where there are dangerous goods and machinery. Should an intoxicated person attending the function directly, or indirectly, cause an accident resulting in the injury of an employee, you could well find yourself liable under the Health and Safety in Employment Act 1992.