Waitaminute–didn’t you dissect a frog not too long ago? Do you remember that heavy-duty shell that they have? No? Good, because if it had a sturdy shell, you were examining a turtle and no one told you.
The turtle has a marvelous defense against human feet, but the frog does not. If you do indeed step on the frog, its only hope is that you are very, very small.
A frog (or a toad, which is a type of frog) may have a chance to evade your pounding feet if it is not too cold and if it jumps in the correct direction fast enough. It would be very serious if you stepped right on it on a hard surface. The frog might have a chance in a soft mud puddle if you instantly shifted your weight, realizing your misstep.
Possibly there are some tiny tree frogs that could use their tongue to get away, but it’s better not to imagine that situation because it would probably also involve poison and a jungle emergency scenario.
The short answer is that nothing good will happen to the frog if you step on it.
It will not magically shape shift its way out from under your feet like a cat and vanish into the next room leaving a horrible Doppler screech. It will not make a gurglesy dog yelp and then apologize with its eyes.
It will not hold a grudge, but it will probably just be a puddle of former frog.
Try to avoid stepping on them–they need all the mercy they can get.