You might face a situation where some standard or 3rd party controls being used in your application across various forms/windows. In any case you have a situation where you don't close any of the forms where those controls are being used, instead you hide them for some reason. These controls might cause a flickering effect on other forms particularly when the hosting machine is slow.
Recently I came across such a situation where an opensource listview control was causing a black ribbon like effect on other screens. After trying various ways, I was able to avoid it by disabling the visibility of it upon the Form Leave / Page Leave event.