AutoCAD 2007 has a "Selection Preview Effect": lines get thicker when you go over them with the mouse pointer.
Bug:
(1) make a block
(2) move mouse pointer over block: lines become thicker
(3) navigate with SpaceNavigator while mouse is over block
(4) stop navigating
(5) move mouse away from block
(6) bug: lines are still thick, until you select the block and press escape.
Release version 3.1.8
Driver version 6.1.7
Firmware version 3.17
Edit: It's not only with blocks, also with simple lines,...

