Online questions about power supplies

bad design

Sounds like a homework question to me.
Bad design, poor quality materials and workmanship, it being strained too far or not strained enough and possible dirty power it receives from the wall.
If you want a practical example of this, go to your local computer market and buy the cheapest beige PSU of the highest wattage you can find. Bust that open, you will see what makes a bad PSU tick.