A post in Blender Online - Vietnam's Blender User Development forum - (https://www.facebook.com/groups/blenderonline/permalink/2077823505829617/?comment_id=2077900142488620¬if_id=1526824146740746¬if_t=group_comment_reply) indicated that there is an error in GUI drawing of mirroring edge loops after performing a 'Ctrl+Z' (undo). I've tested this on Linux on both 2.78c and 2.79.4 and could reproduce the error in both versions.
How to see it:
- Open Blender
- Select the default Cube, press NUMPAD_1 to Front view, NUMPAD_5 to Orthographic projection.
- Tab into Edit Mode
- Press 'Z' to Wireframe mode
- Ctrl+R to create a vertical edge loop, LMB to confirm middle position, RMB or Escape to stop sliding motion, placing the loop cut at the centre position, dividing the cube in half.
- Press 'A' to unselect all.
- Press 'B' to box select only the left side of the cube, press 'X' or 'Delete' to remove all selected vertices. The right half should remain intact.
- Add a 'Mirror' modifier, leave all default settings (can optionally select 'Clipping' but it shouldn't make any difference).
- Ctrl+R to create a vertical edge loop, LMB to confirm, drag it to the right, release RMB to confirm. Notice a dark grayed line mirror of this edge loop on the left side.
- Ctrl+R to create a horizontal edge loop, LMB to confirm, drag it to the top, release RMB to confirm. Notice a dark grayed line mirror of this edge loop on the left side.
- Now press Ctrl+Z to UNDO the last operation (horizontal edge loop creation). All grayed lines on the left disappeared.
- Tab out of Edit Mode, back to Object Mode. On Properties panel, Object Tab, Display area, turn on 'Wire', 'Draw All Edges' and you should see the vertical line on the left reappear, indicating the data is not lost, but the display in UI has erroneously cleared the drawing lines.
Image before pressing 'Ctrl+Z' to UNDO
Image after pressing 'Ctrl+Z'