I had a board with 3 LEDs, two driven by an AtXmega325e and one simply connected to the rails. The rails are 3.3V, the series resistors 70ohms.
All LEDs stopped working at once, yet the rest of the board (OpAmps, ADC, UART) seemed completely functional. After replacing the LEDs, the new ones worked as intended again, so I must have damaged the LEDs themselves somehow. I am not sure whether they were oriented correctly, but they did not light up (as they normally do (albeit dimly) when measuring the diode drop) in either direction. The diode drop function of my multimeter measured the following (disconnected from the power supply):
forward backward 1.7V 0.6V driven by uC 1.6V 0.6V driven by uC 0.5V 1.2V power rails directly
The most confusing thing: After dismounting the LEDs they actually worked again. The two IO pins have been in all possible combinations (both on, etc.) but the LEDs never lit up. I lost the datasheet/name, but they are generic orange surface mount LEDs, with a max. current of 20mA (the names in the schematic are wrong). The solder joints looked completely fine, I also find it improbable that all three would be defective.
Do you have any idea what could cause this strange behaviour? How should I investigate this further?