The down side is, since you didn't overcook the pizza, you'd no longer be motivated to build the time machine, thus would not send a good pizza back in time. Meaning that your past-self would then overcook a pizza, making you decide to build a time machine to send yourself a good pizza back in time. Then, since you didn't overcook the pizza, you'd no longer be motivated to build the time machine, thus would not send a good pizza back in time. Meaning that your past-self would then overcook a pizza, making you decide to build a time machine to send yourself a good pizza back in time.
Next thing we know, Apes rule the planet and weird, underground mutants worship a nuclear dildo-shaped bomb that blows up the planet.
All because of your precious, overcooked pizza. I hope you're happy, you nancy.