Check mistakes
A mistake is a fill that breaks a rule. The board shows it: a repeated digit, two stars that touch, three 1s in a line.
Undo the last few fills and look at that group again. Check mistakes is there to find the clash, not to keep a score.
You can restart the same puzzle. The cells that were given stay. Your fills go. The solution is still the same one.
A mistake is ordinary. Fix the group and keep going. Time stays private. Nothing on the site treats an error as a verdict. Home is the menu if you want a different board.