There are several rumours about icing.
I remember in my active time we had an engine test run in winter time (about 4°C and foggy outside). The mechanic in the cockpit noticed a fire warning and shut the engine down. While there was no smoke visible we entered the testing hall and removed the intake covers (which should avoid FOD). My collegue had a look into the intake and decided to climb into it. Well, he was one of the craziest guy I met. A second later we heard "Aaaaargh" and we could not see him anymore. When we asked him what happened he replied: "It is iced in here!" As a prove he threw big pieces of ice out of the intake.
As this happened ten or even more years ago I am not sure about the time the engine was running. But I do remember we did not start the adjustments. So the engine must still have been in its "warm-up" phase.
Weather conditions are the most thing I am afraid of when flying 
