Only negative feedback
As Insead Knowledge states, feedback should never be solely negative: this would only be demotivating. If you want to make it clear to an employee that they are doing something wrong, always mention also some good practices and traits which they should further develop.
Hypocrisy of the manager
A manager should never preach water and drink wine. For instance, if you tell employees they need to organise their time better but you yourself are late for every meeting, you will never be taken seriously. You have to serve as a role model and follow your own advice that you give to employees.
Inability to tell things how they are
Some managers beat around the bush when giving negative feedback, being unable to express clearly and directly what it is the employee should work on. The result is that the employee leaves the meeting feeling confused. If you want someone to make changes, you have to state this clearly without any sugarcoating.
Inability to set a time frame
Any feedback session should eventually lead to a certain plan of how the agreed targets will be reached. This plan should also have a fixed time frame that manager and employee will adhere to.
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