Some companies are paying up to $10,000 per tweet for an endorsement from a high-profile celebrity. But a study by University of Colorado at Boulder shows what we already know: tying in with a celebrity is risky business. "The overall message to marketers is be careful, because all of us, celebrities or not, have positives and negatives to our personalities, and those negatives can easily transfer to a brand," says Margaret C. Campbell, an associate professor of marketing for University of Colorado at Boulder. Six examples at the jump. Yes, Tiger is one.