A man is prone to stumble when walking, and a horse is prone to stumble when galloping.

解释 ( paraphrase )
人走路时难免有脚没踩稳而摔跤，马在奔跑时难免有蹄子踏空而跌倒。
这条谚语说明任何一个人都难免有失误或犯错误的时候。重要的是有了错误应该认识和改正。

This proverb tells us that one can’t avoid making mistakes; what is important is to recognize and correct one’s mistakes.

失足：行走时不小心跌倒。比喻人堕落或犯严重错误。
         To stumble when walking, often used metaphorically to mean to make a serious mistake or fall into perdition.

造句 ( example )
“人有失足，马有失蹄”。考试时我检查了很多遍试卷，结果还是没能拿满分。