Nothing like having an 11 hour carve get ruined in the last few minutes cause of the machine throwing the cut path off!

As the machine was making it's cutting pass on a 10"h x 36"w project, I watched and noticed that it wasn't following the path correctly. It appeared that the cut path had been shifted to the left about half an inch. I immediately aborted the project and attempted to cut the object out myself with a coping saw, but the damage had already been done.

I double checked the file, thinking I had inadvertently moved the cut path during a move or resize, but it was spot on.

Any ideas as to why the machine would have decided to shift the cut path over on it's own?