There’s a few “petit pain au chocolat” which is the best kind of technically correct. And right next to it the absolute most wrong: “petit pain”.

3 big ones for all cases:
-texture. Frozen stuff tends to go a bit soft or mushy, so if you want crispy/roasted/whatev. It’s going to be at best difficult, sometimes not doable.\

• cooking time. If you don’t want to think too much about it, thaw before cooking, otherwise increase the cooking time to some degree. (Depens a lot)\
• watery. Frozen vegs release more water when they defrost, you need to consider that with the rest of the thing you are cooking. Thawing before cooking helps to an extend, but not always.
  Also some recipes ars meant to work with frozen ingredients, that takes care of the guesswork.