We once had a top outboard bearing fail on a C172N.
This was on the right hand flap during a demonstration to a student by an instructor of flap operation. The motor kept retracting till it broke the flap spar. Result was outboard section of the flap was half extended, the inboard half retracted as was the (cable operated) port flap. The aircraft was just controllable - how it would have been if the flaps were extended during the turn on to final, who knows...
I sent in an MOR but the CAA said that was only the 3rd occurrence so wasn't aproblem. I suggested that the rollers were removed every 3 years to be properly cleaned, inspected and relubed - they are needle roller bearings. Smearing grease on flap tracks does nuffin'!
At the paint shop, we often find Cessna flap rollers in a shocking state when the flaps are removed and have to be replaced.