A manager who wants to show his staff they can come and share their ideas or work problems with him often says: "My door is always open for you."
In reality, however, this makes it clear that if someone wants to discuss tricky issues with such a manager, they have to come and enter his territory. It also sends a signal that the manager (often unlike subordinates) has a door that he can close whenever he wants.
In short, managers often can't see how difficult it may be in reality for their subordinates to approach them in order to deal with problematic issues. Moreover, those subordinates who do dare to come are frequently punished as a result.
How can you operate more effectively so your co-workers may be frank with you?