I'm working in an Architectural drawing in Model Space using ACAD 2008 SP1. When I zoom in past a certain point, which isn't very far, the UCS icon changes to a horizontal line with an 'X' at the right end.
It is similar to if you were to rotate the UCS -90 degrees except it doesn't show the Z-axis or the circle at the left end. Zoom out and it is fine. It is the same result if you zoom-window or scroll-zoom.
If you type PLAN whilst zoomed it it simply zooms to extents so I would say it isn't flipping the UCS. I also have Dynamic UCS turned off. There are no 3d objects, or any items with a Z coordinate in this drawing or any XREFs.
This has me beat, can someone shed any light? It is annoying because it makes it hard to trim/extend.
I've attached a screen shot, to give an idea of the zoom level, the distance between the orange and yellow line is 90mm.
I know that this is an old thread, but we recently found this problem occurring in our drawing. It seemed that the coordinate system was very large (i.e. real world coordinates in millimeters). Once we moved the detail to lower coordinates (i.e. closer to 0,0), the problem seemed to go away. Hope this helps someone!