Why Your Converted Recipe Came Out Wrong
The most common causes are using a generic volume-to-weight conversion factor that doesn't match the specific ingredient being converted, scaling leavening agents and spices linearly with the rest of a recipe when they don't scale the same way, and not adjusting cooking time or pan size when a recipe's total quantity changes significantly.
A recipe that turns out wrong after conversion or scaling almost always traces back to one of a small number of specific, well-understood pitfalls rather than the underlying math being flawed.
Using the wrong ingredient-specific conversion
Applying a generic "1 cup = X grams" factor without accounting for the specific ingredient is the most common source of a converted recipe going wrong — flour, sugar, butter, and liquids all convert differently between volume and weight, so a single universal factor applied across all of them introduces real error.
Scaling leavening agents and spices incorrectly
Baking powder, baking soda, salt, and strong spices don't always scale linearly with the rest of a recipe — doubling a recipe doesn't always mean exactly doubling these ingredients, since their effect can be disproportionate at larger quantities. Adjusting these to taste, rather than strictly following the linear scale factor, generally produces better results.
Forgetting to adjust for pan size and cooking time
Scaling a recipe's ingredients up or down doesn't automatically scale the correct pan size or cooking time — a doubled batch in the same size pan bakes differently than the original, and often needs a longer cooking time or a larger pan altogether. This is a common reason a correctly scaled recipe still doesn't turn out as expected.