Unwillingness to delegate work
As The Muse states, many new managers subconsciously believe only they themselves are able to do things to the required level of quality. They are then reluctant to delegate tasks and become increasingly overwhelmed. An ability and willingness to delegate work is one of the most important and also most difficult tasks of any new manager.
Perfectionism and focusing on details
Managerial newbies often feel they need to have everything that is going on in the team under control. Thus they sometimes focus on details and insist pedantically on pointless rules rather than concentrate on overall results and giving employees sufficient breathing space.
Unnatural management style
New managers, in an effort to appear sufficiently professional, often read numerous books on management, follow expert advice, and ultimately try to manage in a style that for them is not natural. Every manager needs to find their own style that corresponds to their personality instead of trying to copy other people.
Trying to please everybody
In an effort to get along with their subordinates (as indeed they should), inexperienced team leaders sometimes end up promising things they cannot deliver. New managers should avoid this.
Lack of decisiveness
The last mistake to mention here is a lack of decisiveness. Some new managers do not feel sufficiently self-confident to make quick, confident decisions, which sometimes ends up having a negative impact on the results or functioning of the team.
-mm-