Sometimes an application prints characters outside of a particular window, or leaves artifacts--lines or other remnants from the window--when it is cleared from the Workspace. This is sometimes called "window damage." To clear these artifacts, you refresh the Workspace. This causes the entire screen, and all your applications and icons to be repainted.
To refresh the Workspace, choose Workspace -> Utilities -> Refresh.
After a few moments, the artifacts are cleared from the Workspace.