How bad is it to boil instead of simmer?
Really Bad. A simmer consists of small yet constant pockets of bubbling and a few wisps of steam (roughly 180°F). A boil produces constant steam, with large bubbles quickly rising to the surface (212°F). Boiling destroys the protein in meat, drying it out. However, a boil followed by a cold rinse is best for colorful vegetables like green beans, which can fade when simmered. The takeaway? Closely follow the method the recipe calls for.
How bad is it to cook before the oven is preheated?
Somewhat Bad. This shortcut could raise your risk of food-borne illness, especially if food has been out at room temperature for a while. When food is in the danger zone of 40°F to 140°F, bacteria multiply quickly. To be safe, it’s better to wait until the oven reaches cooking temperature. This is how you should be cooking chicken breast.