Foreseeability and Causation
In order to sue someone for damages suffered, regardless of the legal theory (negligence, strict liability in tort, warranty, etc.), you must show that the harm suffered was legally caused by the wrongful act (whether the act is failure to use reasonable care in the case of negligence, production of a defective product in strict liability in tort, or failure to be a merchantable product in a warranty claim, etc.).
Generally, causation is present, and damages are legally caused by a wrongful act, only if the injury caused in fact by the defective product or wrongful action is a foreseeable consequence of the product's defect or a wrongful action.
For example, if the vending machine harms someone because its electromagnetic fields disrupt someone's pacemaker in a country where pacemakers are unknown, the vending machine's emission of EMFs would not be the legal cause of the heart attack caused by someone's pacemaker failure when exposed to the EMFs, since it was not a foreseeable consequence of the vending machine design, even if it was a factual cause of the heart attack.
Whether the possibility that someone who is wet would come into contact with a vending machine and be exposed to its exposed wiring causing the wet person to be harmed by an electric shock is foreseeable and hence a legal cause of the injury suffered is the kind of question that judges and juries resolve on a case by case basis considering all of the facts and circumstances including, for example, testimony from the people who designed the vending machine and expert witnesses. I could imagine that question being reasonably resolved either way in the absence of hearing the evidence myself, and an appellate court would probably uphold either resolution of this question of fact if there was any evidence in the record of evidence presented at trial to support the conclusion reached on this issue.
An overview of these issues can be found here.
Superseding cause a.k.a. intervening cause
The notion that an act is a superseding or intervening cause which breaks the usual chain of causation of an otherwise foreseeable harm is closely related to foreseeability. Wikipedia gives the following accurate definition and illustration:
In order for the intervening cause to be deemed superseding and
relieve the tortfeasor of liability, both the act/event and the injury
must be unforeseeable. For example, assume that contractor A was
responsible for fencing or marking a hole in the ground and
negligently fails to do so while contractor B is working in the hole.
Then, a driver—who negligently failed to take his medication before
driving and therefore does not see clearly—drives into the unmarked
hole and injures contractor B. Contractor A will still be liable for
the damage to contractor B despite the driver's negligence in not
taking medication. This is because, even though the negligent act of
the driver is not foreseeable, the fact of injury by a driver is
foreseeable (i.e., a car falling in because there is no guard).
In the context of your example of a vending machine's wiring causing injury to someone who is wet, the question is whether the person being wet is something that breaks the chain of causation from a vending machine with wiring that is not insulated fully causing injury depends upon whether it is foreseeable that someone wet would use a vending machine. If so, it is a foreseeable event and not a superseding cause.
Given the vending machines are commonly sold for use near locker rooms, hotels with swimming pools, recreation centers, and outdoors in places where heavy rain is not unusual, it is hard to imagine that being wet would be a superseding cause so unforeseeable as to break the chain of causation.
The formal definition doesn't quite capture how disruptive and surprising (and usually intentioned) something must be to be a superseding cause. The heartland of superseding causes are things like criminal and suicidal conduct.
For example, if someone dies because they intentionally tip the vending machine over upon someone either with an intent to kill that person, or with an intent to kill themselves, and that intentional conduct is what causes the person to be exposed to the vending machine's wiring which is the straw that breaks the camel's back and kills the person, that would be a typical superseding cause.