Common mistakes in Scandi Christmas decor include random accumulation of decor instead of coordinated abundance, ignoring a tight color palette, and failing to layer lighting properly. These missteps lead to cluttered, incohesive spaces. To avoid them, stick to two to three motifs, use a consistent color scheme like red-and-green with an accent, and light the room in layers with ambient, task, and accent lighting for a warm, intentional festive ambiance.