What's the biggest GST filing mistake you've seen in a small business?

I've noticed that many small businesses either file GST returns late or make errors that lead to notices, penalties, or ITC issues. From your experience: What's the most common GST filing mistake? How can businesses avoid it? Any tips or checklists that have helped you stay compliant? I'd love to hear real-life experiences from accountants, business owners, and tax professionals.