Some animals that are poisonous have bright markings to make predators wary of them. (Right?)

And some animals that aren't poisonous also have these bright markings so that predators are also wary of them also (even if they're not actually poisonous)

How did this happen?

1) Why do poisonous animals need to warn off predators? If they are poisonous, any preadator that does eat them will die and their species will thrive as a result of their predators dying off.

2) How do the fakers never get found out? Like one really hungry preadator has to eat some faking colourful animal out of desperation and then finds out that the animals are not only poison-free but totally delicoius. Then eats loads of them a and then thrives.

3) I am definitely stealing from another DiSer's routine. Who am I thinking of?