If you consider all consequences and do a task thoughtfully, take your time and watch what you are doing, you are being careful. Can you ever be too careful? If you're a stunt man, yes. If you're an accountant, no.
Being cautious has always been wise, and the word careful has been an admonishment since before the Tenth Century. In Old English, the word cearful meant "mournful" or "full of care or concern," the second meaning not too far from our current definition, that of "using care." While we are often warned to be careful about things in our life, Ralph Waldo Emerson observed that "A man is usually more careful of his money than he is of his principles."