1. Putting cards into sleeves at an angle
2. Using old or dusty penny sleeves
3. Wiping cards with cloth (scratches foil!)
4. Over-tight top loaders
5. Pressing down on the card when sleeving
6. Storing cards flat under heavy items
7. Touching holo surfaces with fingers
8. Taking cards in/out of sleeves repeatedly
These mistakes cause real surface and corner damage.