A few years back there were a number of incidents, some involving evacuation on the runway, when urea prills were ingested into the engine as reverse thrust was applied.
The aircraft that seemed to be affected were the then relatively new B733/734s. Inside the aircraft I believe there was a light misty smoke with an acrid smell when it happened.
Not a problem these days, certainly in the UK, because prills are no longer used for environmental reasons.
Just in case anyone hasn't come across urea prills, they were pellets of urea that absorbed moisture on the runway surface and then mixed with the water to lower the freezing point. If you didn't get them down early enough they just sat on the icy surface and blew away or got ingested in low slung engines.
