'Careful' in the Bible

According to the [remarkable] grace of God which was given to me [to prepare me for my task], like a skillful master builder I laid a foundation, and now another is building on it. But each one must be careful how he builds on it,

Only be careful that this liberty of yours [this power to choose] does not somehow become a stumbling block [that is, a temptation to sin] to the weak [in conscience].