One of the most common mistakes by junior animators is to animate a character taking just one step.
The reason this doesn't work only becomes clear if you try doing it. Take a single step and...guess what...you'll take a second step.
The fact is that everyone takes two steps. Life is like a dance, we are constantly using our bodies in a one-two one-two motion. It's tempting to animate a character taking a single step because it seems to make sense. After all, so much of what we do with our bodies is automatic; done without thinking. But, if you try taking a single step, you'll quickly realise why you need a second catch-up step to maintain your balance.
The reason this matters is because, as animators, we need to make our work feel lifelike and believable. If we don't get it right, it feels weird, and the audience will notice.