I remember an incident a few years ago ( October 30, 2007 ), a Romania Mig-21 Lancer C was struck by a UFO.
There are some videos on youtube:
http://www.youtube.com/watch?v=7N_IAcjTpKk
I will translate the important parts of the videos for I am sure you don't speak Romanian.
> The mig was flying at 6500 m ( 21,325 feet ) when it struck a UFO ( OZN in Romanian );
> The canopy was broken;
> There were 2 solid objects numbered on the video recording;
> The mig landed safely;
> The authorities have confirmed the event;
> The possibility of bird strike and ice was excluded;
> And they said that the video recording is bad because of the collision;
> In the end this event is studied at universities worldwide !?.
I don't realy know, it might be ice from white vapor trails caused by airliners but that is a wild guess.
And it might be part of the mytery of life ......, what do you think?