Another thing that can happen if you put 5V on a 3V3 chip's input pin(s) is what is termed "CMOS latch-up"There is a page for this : http://playground.arduino.cc/Main/I2CBi-directionalLevelShifter
It would matter which can handle more current, i imagine neither are current limited so the 5v will source current to the 3.3v until one gave out
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