The IDE sometimes disables the editor’s undo system to do some text extractions. In combination with a disabled “Keep trailing blanks” and/or an active “Optimal fill” this could cause the undo system to become out of sync with the editor content. IDE Fix Pack turns on/off those options when undo is temporarily disabled so that no unexpected automatic editor buffer changes happen. Especially Delphi 2009 is affected by this, newer versions fixed (almost?) all of these gaps.