According to me one of the greatest challenges a Project Manager faces are that of communication as mentioned by most above among the many other challenges that are faced by a project manager. Sometimes a lack of communication within the project group members and between the PM and the group members can lead to a lot of problems due to miscommunication which could include missing deadlines or not addressing a problem that would be encountered on the project well on time. This would also include a lack of responsibility in informing the group members the necessary information via e-mails etc. In other cases a sense of over-communication from the PM as was mentioned by @manolo in his post above would also be problem which would lead to a feeling of superiority and would not allow the group members to work as efficiently as they would otherwise.