An expensive wiper blade.
Following is a Japan Air Lines 747 crew report.
"We saw the captain-side wiper blade bending upward about 10 minutes after takeoff from NRT. Rain was forecast at YVR at our ETA. After consulting the Flight Operations Center and the Maintenance Operations Center, we decided to return to NRT though we had to dump 48,000 lbs of fuel.
The blade in question had replaced the old one that stopped working after riding over the stopper while the aircraft was approaching NRT on the previous leg."