Why a Dish Came Out Underdone After Adjusting the Temperature
An underdone result after lowering the oven temperature usually means the cooking time wasn't increased enough to compensate — a common mistake is applying only a small time increase (or none at all) when the temperature drop was actually significant, or not accounting for the fact that a dense or thick item needs proportionally more additional time than a thin one at the same temperature change.
This specific problem is directly connected to the general adjustment principle covered in the temperature-change guide, focused here on the most common way it goes wrong.
Underestimating how much extra time is needed
A modest time increase that doesn't match the actual size of the temperature drop is a common cause of an underdone result — the rough 25%-per-25°F guideline (covered in the adjustment guide) is a reasonable starting point, but a larger temperature drop needs a correspondingly larger time increase, not just a token adjustment.
Not accounting for item thickness or density
A thick or dense item (a large roast, a dense cake) is more sensitive to a temperature reduction than a thin item, since heat needs to penetrate further to the center — the same percentage time adjustment that works for a thin item may not be enough for a thicker one, which needs a proportionally larger increase.
Fixing an underdone result and avoiding it next time
If a dish comes out underdone, continuing to cook it (checking doneness incrementally, as covered in the adjustment guide) rather than assuming it's finished based on the originally estimated time salvages the current dish — for next time, checking doneness more conservatively and building in extra buffer time for a lowered-temperature adjustment avoids repeating the same shortfall.