The BCM2835 is directly connected to signals present on the GPIO connector. Therefore, when you connect to the GPIO pin header, you are directly attaching to the SOC, and an overvoltage over-voltage or overcurrent over-current condition can seriously and irreversibly damage or destroy the SOC and render the Raspberry Pi useless.

These Two conditions cause particular problems:lead to disastrous consequences;# Grounding a pin which is expected to be an input when it is configured as an a high output(3.3 volts, often written as 3v3), either through the GPIO configuration or the PinMux configuration. This will cause as much a large current flow, probably damaging the Raspberry Pi. Likewise, connecting a pint which is expected to flow be an input to 3v3 when it is configured as a low output (0v) will have a similar effect.# Connecting any pin to a voltage greater than 3v3 or less than 0v. To fully protect a pin, attach a moderate resistance (220+ ohms) and a reversed zener diode to ground rated just above 3v3. It is available also important to protect the Pi against:# [http://en.wikipedia.org/wiki/Electrostatic_discharge Electrostatic discharge] (however, it should be noted that the Pi is reasonably ESD-tolerant)# Excessive current draw (including total current drawn by all attached devices)# Over/under-voltage from the power supply,