In Material 3, if a user attempts to create a MaterialStateBorderSide override for the checkbox side property, it does not take effect if specified for error states.

See https://dartpad.dev/?id=fbe8db2ca18aedb667964643c29a667e&channel=beta for a working example of this issue.

It works correctly for other properties (fill color, etc) as those fields specifically check for isError in the widget and add it to the states, e.g this current example for checkbox color:

final Set<MaterialState> checkStates =  widget.isError ? (states..add(MaterialState.error)) : states;
final Color effectiveCheckColor = widget.checkColor ??  checkboxTheme.checkColor?.resolve(checkStates) ??
        defaults.checkColor!.resolve(checkStates)!;

A similar check for error needs to be added to the _resolveSide method.