Those are all things that take components on the board past their absolute maximum ratings - some of them might take a while to do damage, and you might be lucky, but its definitely worth avoiding all them !
People for instance often short out output pins and get away with it - or end up with only slight damage (one pin doesn't drive fully high or fully low - I've got one Arduino Pro Mini like this for instance). In general exceeding the 40mA limit on pins will lead to permanent damage as the output FETs overheat.
The barrel jack is protected against reverse voltage but the Vin pin is not - hence method #4
So yes - take care, and if you can't take care (perhaps you are teaching a class!) consider a ruggedized version to reduce the accidental releases of the "magic smoke".