"...
1. Air bubbles
2. Cookie shells that separate from the cookie itself
3. Excessive coloring
4. Substituting ground almonds for pistachios
5. The almond extract issue
6. Stale macarons
7. Macarons that are too big or too small
8. Almond flour not ground finely enough
9. Too much sugar
10. Too little filling"
(Source and more details: Link)
