Great warnings from Piper below. I would also add to his list of "loose wires" the following along with an example or two:
First, a lack of concern for compromise on the creation account in Genesis 1-3. For example, a number of reformed leaders accept the false notion that Genesis is not a historical account but figurative or allegorical.
Second, a lack of discernment in false ecumenicism. For example, the Manhattan Declaration was signed by many big-name reformed leaders.
Third, tolerating heretics or not calling them what they are, wolves in sheep's clothing. For example, Piper himself inviting Rick Warren to his conference and many treating him like a brother in the Lord, as opposed to a false teacher. Also, NT Wright is treated as a brother by many as well, who is just misguided or confused on the gospel, but he is a teacher of a false gospel. No matter how nice, brilliant or how many orthodox doctrines you hold to, to miss the gospel is to miss the truth that sets you free. He is a wolf in sheep's clothing.
Piper brings out a number of other good and wise admonitions.