External bad habits:

1. Force against force
2. More than 4 ounces of pressure exerted by you or expressed by you
3. Localised arm  and shoulder movement
4. Deep, long or wide stances
5. Fixed legs - disconnected upper & lower
6. Tensed muscles
7. Over-emphasis of the waist
8. Incorrect use of the pelvis and hips
9. Pushing upon impact
10. Aggression

These will all perpetuate an external approach to tai chi.