Why do people get electrocuted by cell phones
Between primary and secondary of the transformer is leakage capacitance and this leakage capacitance is significantly increased by EMI reduction capacitors on switched mode power supplies (SMPSs). SMPSs are the most common type of phone charger used today so, compared with the old power AC transformer there is more of a risk of feeling a slight "tingle".



Notice the 2n2 capacitor bottom right of the picture - this is what Power Integrations recommend as part of the solution to meeting EMC specifications. Basically it connects the low voltage DC output to the rectified input voltage and, of course the rectified input voltage is still wobbling around at mains frequencies (despite it being constant across the main smoothing capacitors).

Here's another circuit from Power Integrations but please note I'm not targetting them as some kind of menace - a lot of designs that don'r rely on the "earth" pin of a 3 pin plug use this technique: -



Here. the capacitor can be seen directly across the transformer (CY1, 2,2 nF) and note that it is also labelled "Safety" and this implies it must be a type of capacitor that fails open-circuit.

So this can lead to a tingling sensation should you touch the output pins and hold onto something that is regarded as earth. Not dangerous of course but a little disconcerting.

It gets worse. In the design below the output is directly connected to the neutral return wire of the AC mains: -



There is absolutely no isolation here at all BUT this type of supply is not intended to be used as a charger. The Digi-key page I stole it from said this: -


  Design is for applications where an off-line power supply without
  isolation between input and output can be tolerated


In other words it's a design for something like a set-top box where there may be other measures taken (in the rest of the electronics) for isolation of ports such as antenna input and scart output. But what happens when some idiot somewhere finds the circuit and decides they can make these as chargers?

But the other problem that can arise is "smaller" distances between the lethal AC side and the supposedly "safe" output. The chargers are getting smaller and a lot are coming into many countries from disreputable sources. Add into the equation humidy and dust levels and you can find that the AC crosses the usual "distance" barriers that are meant to protect life. Bad design basically or inappropriate use of the device.