Even when rewards make mathematical sense, people sabotage themselves:
Mistake #1: Carrying a balance for any reason. "I'll pay it next month" costs you more in interest than you'll earn in rewards this year or next. If you can't pay in full, you're not ready for a rewards card.
Mistake #2: Applying for too many cards at once. Chasing sign-up bonuses ("earn 50,000 points!") might net you $500 in value, but five hard inquiries drop your score 30+ points. You've borrowed points value from your credit profile.
Mistake #3: Changing spending patterns to hit bonuses. You don't eat out much, but a card offers 5% on dining. So you start eating out more—spending an extra $150/month to earn 5% ($7.50). That's a negative return on time and money.
Mistake #4: Overvaluing points and miles. Travel sites and airlines train you to think points have high value. In reality, 50,000 miles might be worth $200-300, not the $1,500 the issuer's marketing claims. Do the math before believing the hype.
Mistake #5: Forgetting about rotating categories. Some cards require you to "activate" quarterly categories to earn bonus rates. Forget to activate in Q1, and you earned 1% instead of 5% for three months of purchases. Track these if you're going to use them.
Mistake #6: Letting fees accumulate. You got a premium card two years ago, earned some rewards, then stopped using it. But you're still paying the $99 annual fee with no corresponding benefit. Review your active cards yearly and close ones not earning their weight.