You likely rised the Vdd supply by the kickback pulses from the stepper
phases, through the kickback protection diodes in the ULN2003. Then the
PIC latched up and started boiling water. You have to provide a path for
the rectified kickback voltage/current to ground. In your case a simple
zener across Vdd and GND would not have been enough. You would have used
something like a dump resistor between the ULN2003 diode common and ground
(and not connect the diode common to Vdd).

Peter

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