Another thing that can happen if you put 5V on a 3V3 chip's input pin(s) is what is termed "CMOS latch-up" - this will cause
the whole chip to conduct heavily and get very hot, and can only be stopped by removing the supply. It is cause by over current
flowing through the input protection diodes and causing a parasitic (unintended) bipolar transistor action.
You'll often see "latch-up immunity" specified in datasheets, and many chips (especially nowadays) are designed to prevent
this happening unless extreme currents are present on the inputs (latch-up usually destroys the chip and can melt nearby components!).
These days it is a rare thing to experience (although I have triggered it once in the last few years with an ADC chip without
any apparent permanent damage - its surprising when a chip with 10mA current drain rating to get too hot to touch!)
You have to be careful to avoid significant currents due to over-voltage (or under voltage) on inputs - a 10k resistor will typically serve to prevent ill-effects (the poor-man's level shifter?). The best solution is a proper level shifter thoughhttp://en.wikipedia.org/wiki/Latchup
BTW when you do have a 3V3 chip with 5V-tolerant inputs this means there might be no protection diodes on those inputs, so they may
be more sensitive to electrostatic discharge.