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Can someone explain (in a non-wiki technical way) what reverse current is through an Arduino ? Cheers y'all

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It's current flowing "the wrong way".

The electronics inside the main chip are designed such that the + side (Vcc) must be at a significantly higher voltage than the - side (GND). By significantly I mean 2.1-5.5V higher.

Because of that it is expected that the current will flow through the components in one direction (or in a couple of pre-determined directions depending on settings). Any situation which causes a reversal of that current through the internals is bad, and as called "reverse current".

It could be caused by something as simple as connecting a power supply backwards, or more technical things like the "Back EMF" from an inductive load (motor, relay, etc) entering a pin that is an output and flowing backwards up to Vcc, or "Back Powering" where the chip is not powered but some external device connected to it is powered, and it "leeches" power backwards through IO pins to power the chip.

All of those situations are bad and should be avoided.

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Many thanks for getting back. Other then checking my connections/jumpers are the right way round...how do I "spot the signs", test & prevent a reverse current in a circuit b4 "frying" my chips in the Arduino ? –  Seanie Jun 13 at 9:25
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Make sure all inductive loads have flyback diodes, and ensure that external circuitry is never powered without the Arduino powered. –  Majenko Jun 13 at 9:45
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Adding resistor between the arduino, and signals that might have a higher voltage than that of the arduino. This will help prevent to much current going through the clamping diodes (and break) inside the arduino chip. I can't find the specific value for the clamping diodes but I thinks it's around 1-2mA max. –  Gerben Jun 13 at 13:44
    
Ta, Thanks very much Majenko and Gerben –  Seanie Jun 14 at 9:06

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