Why does backwards compatibility fail?

The most common reason backwards compatibility fails: Adopting a new mental model that explains the anomaly that triggered the change but quietly drops coverage of situations the old model handled well. You feel enlightened because you solved the puzzle that was bothering you, but you've introduced silent regressions — areas of life where your thinking is now less reliable than before.

The fix: Identify one belief you've recently updated. Write down three situations where your old belief gave you a correct prediction. Now test: does your new belief also give correct predictions for those same situations? If not, your new schema isn't backwards compatible — it's just different, not better. Revise it until it covers both the old successes and the new cases that forced the change.

The underlying principle is straightforward: Sometimes you need the new schema to handle cases the old schema covered.