I was just touching off a group of tools and on the last one I noticed my Z position was around double what it should have been (machine position -30ish when it only has 20ish inches of travel).  Machine returns to zero at top of travel, as if the scaling is off.  This happened twice tonight.  The second time I looked in the history and just before I set the last tool offset it said "license error 4".
The only way I found to recover from this issue is to restart Mach.

License error 4 means that the core thinks that it has been modified.  It may be a corrupted Mach4Core DLL causing all of the problems. 

Steve