The real problem on chips with dual power rails if current flows from one rail to another rail on the chips caused by having the wrong voltages for too long, the current flow through the chip will heat the current path possibly causing damage.
Take a chip with a 3.3V rail and 1.8V rail and the chip is designed so that 1.8V should not exceed 3.3V rail ever. Then if the 3.3V rail goes to ground the current will flow from the 1.8V rail out the 3.3V input and to ground. The current flow is only limited by the 1.8V source impedance. The energy from the 1.8V source would then be totally be dissipated in the chip possibly damaging the part.
Its not the voltage that kills but the amount of energy the chip has to dissipate. Too much energy and the chip will melt down.
Hope this makes sense. When talking about damage to chips we also have to include energy over time (power) along with voltage and current.
