Guild Wars 2

Just go to Heart of the Mists and visit the Amulet Vendor.
Jump on table and shadowstep to the floor —> works!
Jump on platform to the vendor and shadowstep to the floor --> does not work!
Jump on platform then on cupboard and shadowstep to platform or floor —> both works sometimes and sometimes not, i. e. depending on where you stand on the cupboard!

Try out running on walls and target your shadowstep directly in front of your feet. —> some walls let you shadowstep on them, others drop you down.

So what is the rule that determines whether a target can be reached or not or isn’t there any rule, i. e. Anet has lost control over their code?

The general rule is that if you can’t walk to it directly without being obstructed then you can’t Step to it. This obviously isn’t always the case, but it’s a good rule of thumb for places you haven’t yet tested yourself in matches or wherever.

Either way, it does need to be adressed, because sometimes simply stepping down a hill will get you taken back to your original spot.