This is another post inspired by a frequent discussion-group topic. The scenario is that someone double-clicks inside an unlocked viewport and suddenly the view zooms to extents. Probably not what you had in mind.
Like a lot of unexpected behaviors in AutoCAD LT, the fix is easy once you know where to look. In this case, it's the variable UCSFOLLOW. When it's set to 0, as it is by default, nothing happens to your view when you change the UCS -- i.e. when you go from layout coordinates to model space. But if it gets set to 1, a change in UCS automatically regenerates a plan view, and zooms to the extents of the drawing.
The only thing to remember is that each viewport has its own UCSFOLLOW setting. So if you have more than one viewport acting up, you need to set USCFOLLOW to 0 in each viewport. (That means double-click inside it and change the variable. Changing it in just model space or just a layout isn't enough.)
Locking a viewport also prevents this kind of view changes -- depending on your setup, that may be a viable alternative too.