12v PSU: Harmless or not?
Harmless? If you drop it on your foot it might hurt a bit. 

But seriously though, harmless is a bit of a relative term. As @IgnacioVazquez-Abrams states, hooking it up to your tongue or sweaty salty skin you will get quite the buzz. Then there is the possibilities of shorts in the PSU.  

You really ought to indicate harmless for such and such a situation. 

Generally we consider 12V to not be a dangerous voltage. But there are a lot of provisos in there. For example, with any voltage source, how much current it can source can be dangerous when shorted.

Another thing you need to consider is if it can get hot/overheat/start a fire.