Race between Power Supplies?

Perhaps this is a hypothetical question, but I'm not sure,which is why I am asking it.

There are now enough 3.3 v. parts around so that I am using them all the time. But I have an inventory of 5 v. stuff to get rid of, like Nanos and ProMinis. So I am combining both types of parts, and implementing two power supplies if necessary.

So here is the question. I have seen chips damaged when the control voltage on some pin arrives at that pin before the power supply for the chip has come up to supply level. If I have 3.3 v.and 5 v. parts interconnected, fed by separate power supplies, suppose those supplies arrive at full voltage at slightly different times. For example, if I brought 12 v. down through a 7805, and then used that to supply an LM1117 3.3 v. regulator, the 3.3. v. supply would come up later that the 5 v.

So, are there any possible real-world consequences to this, or am I just being paranoid?

So here is the question. I have seen chips damaged when the control voltage on some pin arrives at that pin before the power supply for the chip has come up to supply level....

...So, are there any possible real-world consequences to this, or am I just being paranoid?

First you say you've seen damage, then ask if there are consequences...

You have to be careful where power or current is involved (for example a motor driver). In those cases you need to make sure there are no unsafe states.

But in general, you can apply a low-current signal to an input pin, etc., before the circuit is powered-up and there will be no damage.

You use Nanos and ProMinis as examples of 5 V parts. The initial pin state is input / high impedance. The first code to run is the bootloader. It fiddles with pin 13. The bootloader takes at least one second to finish. Is that enough time for the 3.3 V regulator to be ready for business? If yes, then you have nothing to worry about. If no, then including a delay in your sketch is appropriate.

Do you connect pin 13 to any 3.3 V device? If yes, then you will have to stop doing that. Or, replace the bootloader with a version that leaves pin 13 alone.

Thanks, coding badly. Now I can sleep at night.