25 differences I wish I knew earlier!

1. Causality versus Correlation
2. Strategic versus Tactical/ Operational
3. Possible versus Probable
4. Absolute versus Relative
5. Output and Outcome
6. Risk and Uncertainty
7. Experiment and Observational
8. Point estimates (average/mean) versus Distribution measures (median/ variance)
9. Understanding versus Justifying
10. Economic profit and Psychic Profit
11. Average versus Marginal
12. Variable costs and Sunk costs
13. Models versus Reality
14. Reality versus Counterfactual
    
15. Data versus Opinion
16. Credible data versus Non-credible data
17. Process versus outputs
18. Design and Evolution
19. Stock versus Flow
    
20. Internal validity and External validity
21. Random and Randomized
22. Statistical significance and Physical significance
23. Principle and Degree
24. Rule of law and Rule by law
25. Sex and Making love!

Any additions?

Update:
26. Urgent versus Important
27. Efficienct versus Effective
28. Explain versus Explain away
29. Mutually exclusive issues/ functions and those not so...