Easy to kill by backfeeding voltage?

[ligius-]

I had one of these connected to a rPi and it worked fine for about a month. Then I decided to hook it up to an Arduino and did not read the datasheet. I've fed 3.3V into the 3.3V and now it's seemingly dead. I did not put an oscilloscope to it to see if it actually does something, but the leds don't light up anymore and it doesn't draw any current.

I had a second module - hooked it up to an Arduino (just the 3.3V regulated out) and connected the output pin to a buzzer. It buzzed for less then a second and it went dead. Not sure if the buzzer killed it or the 3.3V.

Perhaps it would be useful to have a list of DON'Ts, such as: no voltage on the 3.3V output (without power), no inductive loads, no voltage on OUT... I don't really know what kills these modules. Static?