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As per @marcuswareham, the rather important insulation between input and output. Some low-cost non-OEM chargers have design and/or construction flaws leading to only the barest minimum of basic insulation. E.g. we have seen units with 1mm PCB track separation, which is an unacceptable creepage distance but it works so they do it. Others rely only on enamelled winding wire on the transformer to achieve separation, the inter-winding taping or ELV secondary over-sleeving having been omitted. Even where the design is sound, in cheap units with poor manufacturing QC there could be any kind of assembly error such as the input pin flylead trapped against an ELV solder joint, or the secondary transformer leadout snagged around a live component.
Genuine OEM chargers are in a different league of quality when new, but can still have drinks poured over them, or get dropped down the stairs...
