我看网友的句子就是个祈使句，careful 后省略了介词 about/with/of 等，这种省略多见于口语。what to do 应该是介词宾语。下面几句均出自英语媒体：

Want to Get Rid of Obamacare? Be Careful What You Wish For

A Warning for Leaders: Be Careful About What You Do in Secret

Be Careful with What You Say and Do

Be careful with what you ask the gods for, they might answer your prayers

Be careful of what you do in front of kids – Sweet innocent boy trying to help mum with shopping.

    也有些人使用介词in。我的看法是，学英语关键是应用能力的培养，死抠语法，不是正确的学习方法，能够正确运用，摆脱中国式英语最重要。其实，很多语法分析是毫无意义的，而且会造成Chinglish的出现，因为有些表达，特别是口语表达不是完全符合语法规律的，但又是正确的。我们应该重点放在培养语感上，特别是对初级和中级水平学生。况且，我看有些语法解释纯属牵强附会。个人观点，仅供参考。

同意teacher liu解答观点

以上为一个省略第2人称主语（you)并以动词原形直接位于句首的祈使句

主语：you（省略）

谓语：be careful（注意，小心，当心）

宾语：what you do（你所做的事情）

Be careful what you do

参考译文：注意/当心/小心你所做的事情

(1)be careful +wh-宾语从句

You have to be careful what you say on TV.  

在电视上说话时你必须很谨慎。

(2)be careful +that宾语从句

Be careful(that) she's not just using you.  

当心她不只是在利用你。

 The United States has to be careful (that) it doesn't overplay its hand.  

美国必须谨防高估自身实力。

(3)be careful about /of+wh-宾语从句

Be careful about what you say.
对你所说的话要小心。

 Be careful of what you are doing.  

注意你的所作所为.

(4)be careful of +that宾语从句

Be careful of that horse it often kicks.
小心那匹马--它常尥蹶子。

(4)be careful +wh-状语从句

You should be careful when you are making a promise.
你应当谨慎做出承诺。

Be careful when you cross the road.

过马路时要当心。