Version:            (using Devel)
OS:                Linux
Installed from:    Compiled sources

At present, you can select a "show errors" setting that will highlight any emplaced number that does not match the known solution to the puzzle.  I would like to suggest another style of showing errors.  This should highlight those numbers that are inconsistent within the current partial solution.  That is, we don't display all incorrect numbers, we only display those that are inconsistent with other numbers on the screen.  This allows the user to make a guess and start checking to see how it plays out, while being notified as soon as the guess leads to a bad state.

I'd suggest a three-way state, something like this:

show errors:   NEVER
               FOR CONSISTENCY
               AGAINST SOLUTION