When a result number A is computed and used in the next expression resulting in a new result number B and than undo (Ctrl-Z) is made, the calc shows the previous expression with the result A used, but B is used instead.

Steps to reproduce: 
 1) Open a basic calc mode
 2) type 1+1, press [enter]
 3) result is 2, so now use the result and type +1 after the number 2 and press [enter], result is 3
 4) press Ctrl-Z, the calc shows 2+1, press [enter]
 5) now the result is 4 (WRONG!!)
 6) you can repeat step 4) so the calc always shows 2+1 but the result is always +1 higher.

This bug is definitely critical as the user doesn't expect such behaviour and may rely on the wrong results.