Problem with valve shim on Patrol
 

Ok got a big problem with a TB 4.8 litre Nissan Patrol (Y61 de 24 valve) on dual fuel. DOn't know if you guys even get Patrol but any help appreciated.

THe car backfired (running on gas) which i am not sure if it caused the problem but anyway it has thrown 2 shims from the inlet valve from cylinder 2. THey have been throw out of the bucket/cam followers which has stuffed the cam shaft. Now before i start putting it backtogther etc i could do with knowing why it did this.

Is it related to the car running on LPG or is it a camshaft followup problem i am not sure
I htink the backfire maybe caused the valve to open leaving excess room for shim to fall out, however i am not sure if this would leave enough room for it to do such as thing.

cheers for any help