Changed Statistics while Testing

[kosse]

When I'm testing lesson 1, some words of lesson 2, 3, 4 etc. marked as tested, but I didn't test them at all !!!

[Chris]

I don't have any idea how this could happen.

Are you sure that the said stacks were not selected, i.e. the words were not tested?
Is it reproducible or did it happen just once? If it is reproducible, please send me your file with the described behavior so I can investigate it more.