The "Zit" cream that is given to Alex Rider allegedly will burn through any metal, but is harmless to a person's skin. When trapped in the aquarium with the giant Man of War jellyfish he uses the cream against the metal supports of the aquarium. This is impossible, as salt water aquariums can have no metal in them, as metal can drastically change the PH balance of the aquarium, and kill any creature in it.
In the scene where Alex is on the runway, before he breaks there is a birds eye view, for a few seconds you can see the skid marks that has been left from previous takes.
The Portuguese Man o' War does not kill by electric shock as shown with the little fish. It's tentacles are filled with thousands of nematocysts that contain a venom, consisting of mostly proteins, that causes a stinging pain lasting about an hour in humans. The real danger is that the venom is highly allergenic and causes allergic shocks in a large number of subjects.