Well I first asked the question over at competitive REL, but now I am curious if there would be a different answer here, so:

Romeo controls a Storm Crow (non-artifact) and Juliet controls a Hover Myr (artifact).
Romeo pays 1B and evokes a Shriekmaw, saying “kill Hover Myr”. Juliet immediately calls a judge.
How is this backed up?

Players are more prone to allow take backs in Regular. But we, as judges, don't have to enforce any take back. If a play was legal, but strategically bad, we as judges don't have to intervene. And if Juliet calls us, the chance she wants to allow a take back are small, and I wouldn't do anything to push her to go this way.
So playing Shriekmaw sticks: it was a bad, but legal, play.

The JAR explains us how to fix the actual illegal play (targeting an artifact creature with Shriekmaw's ETB ability target ): “However, if the error was caught straight away and backing up is relatively easy, do so. If you do choose to back up it’s important to reverse all the actions – no partial fixes!”

So I'd ask Romeo to chose a legal target, educate him on what are ETB ability (versus spells) and eventualy on why it's important to read cards properly before playing them. I'd also thank Juliet for involving a judge.