been working on a midi controller, midi cc to pwm. using 3 rgb leds with 1k current limiting resistors on all pwm outputs. usually I have it powered via a 7805 regulator, but while programing I have it disconnected from the regulators voltage, (power and ground) but the LED's stayed on pwm outs, back to the Arduino's ground. I also have 3 pots connected, which during programming are left unpowered so the analog in floats causing the leds to light erratically (expected) but what I didn't expect is for this to cause the arduino to melt down. both smd diodes got hot enough to fall off the back, and the chip is now hot as well. how did I pop this thing? seems like over current or somehow backfeeding voltage somewhere.. suggestions?
It is hard to tell your mistake because everything you said seems to check out. So that means you are missing something you did but didn’t tell us.
A wiring diagram and photograph might help us spot what you failed to spot but clearly you had a situation where excess current flowed. It is possible a short circuit somewhere in your powering wiring.
I would not have expected this during programming. I would not disconnect the grounds when removing the power.