After the emerald heist, as Diabolik is pulling the "mirror" across the road, he is wearing no mask as he begins, then it cuts to a shot with his mask on, then he is mask-less once again as he finishes pulling.
When Diabolik steals the lorry the driver runs after it. Diabolik then drives it quite a distance before he jumps off. After the lorry runs off the road the driver stops to look at the accident. Then Diabolik and Eva drive past the lorry driver without him having noticed that Diabolik had run back to the car.
Diabolik recovers emeralds from the ashes of a cremated body. Emerald, a type of green beryl, fractures and discolors when exposed to even mild flame (thus losing considerable value), and certainly cannot survive the intense heat of a crematorium.
Diabolik steals a 20-ton ingot of gold. The ingot is shown to be about the size of a VW minibus, but, because gold is so dense, a 20-ton block would take up much less space: a little over one cubic metre (36 cubic feet, or a cube-shaped block 39 inches each side).
The picture that Diabolik mounts in front of the surveillance camera during the emerald heist would not be able to substitute the actual picture on the police monitor. Firstly it is taken with a hand-held camera without flash, and secondly after mounting it is only lit from the back and would therefore appear much darker on the monitor.
The gold ingot is lowered into its "coffin" using simple rope. Two strands of rope of this thickness would not be able to handle anywhere near 20 tons. (The lid of the "coffin" is however lowered using steel cables though.) Also gold is quite soft and flexible and should have more underneath support than what is given in the film.
When Ginko looks at the monitors one is showing the explosion outside the tunnel and the train slowly backing away from it. The other monitor is showing the train cruising backwards at considerably higher speed.