Hyundai Sonata fan belt

I have just had 90,000 km service done on my 2003 Hyundai Sonata four-cylinder, which includes changing the timing belt and fan belts. Eight kilometres after leaving the mechanic's workshop the new fan belts broke. The mechanic said they broke because the power steer belt tensioner bearing seized and replaced them, but six kilometers later the car was running rough at idle. The problem was a bent valve, which was blamed on a seized/blocked hydraulic lash adjuster. What could bend a valve? Could it be a timing issue, maybe one tooth out on the belt?