What happens to a Micro-controller if you short VDD and VSS across a GPIO pin?
The chip getting very hot is a typical symptom of latch-up. It can be induced by a test probe slipping or something like that. A common mistake is to apply signal from a signal generator with large output capability before the circuit is powered. It will survive this, typically, until you turn the power on, then you get the sizzle. 

Modern IC chips are resistant to relatively large errant input currents that would in the past cause latchup (tens of mA or more) but at some point you are going to trigger the giant parasitic SCR that lives inside your CMOS chip and then the current is limited only by the power supply capability and series resistance.  

We also see this in space applications where radiation can cause latchup. 

At 600mA you probably fried the chip to some degree and irreparably damaged it . 

Sometimes if you set your power supply to a lower maximum current limit (not much more than your circuit actually requires)  you can survive this kind of incident. In such cases, cycling the power is still required to get the SCR to turn off.