Here they are, the most popular ways to lose money on Google Paid Search (and other too)

Duplicate Keywords: Hard to believe, but they are pervasive! If you have not checked your program lately I strongly suggest you do and on a regular basis. Duplicate keywords occur over time when companies add new campaigns, new landing pages without vetting the keywords against their historical campaigns. And it gets worse when a backbone architecture is not in place.

Broad Match: Broad match keywords are expensive and often poor performers. But the only way to find out if they will work for you is to test and compare.

Click Inflation: The cost per click just keeps going up on paid search. Cost containment is a major and ongoing challenge to small businesses. Constant maintenance is the only solution: make sure everything is optimized, keyword to landing page. Keep those quality scores up and shift dollars to best performing keywords.