Mistake #1 Making Inappropriate Pre-Employment Inquiries
"That's an interesting accent you have. Where were you born?" "Do you have kids? If so, will you have any daycare problems?"
Seemingly innocent questions are inappropriate during interviews and are a primary source for claims of discrimination. Such questions also don't help identify a candidate's knowledge, skills and ability to do the job they have applied for.
Courts generally assume that if you asked a question, you intended to use the answer as a factor in your hiring decision. Therefore, any questions about or referenced to protected categories like sex, age, race, national origin, or religion can later be used in court in a discrimination claim. Use behavioral based interviews and stick to questions based on the job description to ensure you are hiring by asking the right questions.
Mistake #2. Delivering "Dishonest" Evaluations or Not Giving Feedback At All!
"I gave her a "good" rating even though her work is poor, because I think a "poor"rating would be demotivating."
Many managers and supervisors avoid the discomfort of delivering a review that indicates poor performance and instead avoid the issue with a "satisfactory" rating. As a result, many legitimate actions taken against an employee based on poor performance can be questioned because the performance reviews are positive.
Other managers don't give feedback at all because they lack the skills or don't have the time to give employees honest feedback about their current strengths and areas for development.
Giving feedback on behavior, whether reinforcing or developmental help employees stay on task and grow in their roles.
Mistake #3. Not Communicating What They Know.
Knowledge is power. Managers generally have more information about the business, its direction and significant upcoming events.
When managers don't share information with employees it feeds the rumor mill and demotivates employees. Share what you know, when you know it so employees stay engaged.
Mistake #4. Making Rash Disciplinary Decisions
"That's it, I've had it, you're fired".
Ultimately, terminating an employee may be the right course of action but doing it when you are angry or don't have back up data is not the best way. If you fire when you are angry you are likely to increase the fired employee's desire to sue.

In addition you should never fire without carefully reviewing the circumstances with HR. Managing performance on an ongoing basis will help prevent rash terminations and help you when you have legitimate reasons to fire an employee.
Mistake #5. Forgetting You Are The Manager
Managers may innocently invite the team for drinks and dinner for team building. Inviting employees out for a drink after work may seem a nice gesture, but subordinates may view it as an order.
Repeated requests for dinner or drinks may be viewed as coercion or harassment. Supervisors and managers are agents of the company, and when they engage in behavior that may be considered harassment, it puts both the manager and the company at risk for lawsuits.
Team-building is a great idea and done right can build morale and increase productivity.