Most integrated circuits have a "P" doped substrate that is tied to the most negative power supply rail The components of the the integrated circuit are "N" doped regions in the surface of the "P" substrate. When any pin goes more negative than the negative power supply rail the PN substrate junction (also known as a parasitic diode) forward biases and allows a large current to flow. When the current rises to about 1 Amp (in most parts), the bond wire fuses causing an open circuit.
Product datasheets normally have an Absolute Maximum Rating for pin voltages that limits them to -0.3V with respect to ground, or to the negative supply if one is required. Keeping the pin voltage within -0.3V of the most negative supply rail prevents this PN junction from conducting enough current to cause damage. An exception to this rule is found in automotive regulators, which often have built-in protection for negative input voltages. Check the product datasheet to find the pin voltage limits for the device you are concerned about.
