what happens if I target creature with shroud in tournament

[Icey]

what happens if I target creature with shroud in magic tourney?

[Twanos]

When you announce that you are targeting the creature with the spell the game recognizes this as an illegal action and says you can do it so you are refunded any mana you used to cast the spell and it is returned to your hand. There would be no corrective action taken by a judge unless it started happening alot.

[Aoe]

There would be no corrective action taken by a judge unless it started happening alot.

... or if after the spell returns to your hand you cast it targetting the SAME creature with shroud AGAIN

[Magmar]

you should play wrath of god instead because that is legal play to kill the creature with shroud

[seba1983]

not every deck can run wrath of good. also global removal is not always what you want. on top of that there are formats where a 4 mana removal often is just too slow.

[Arix]

More technically, an error like this falls under "Game Play Error - Game Rule Violation". It would give you a caution at regular REL, and a warning at competitive and professional RELs. If the infraction is discovered in a reasonable amount of time (usually within one turn cycle), you can just back up and rewind to the point just before the illegal action occured.

That said, if it was caught immediately ("Cast Doom Blade on your Sphinx of Jwar Isle." "No, it has shroud." "Oops, my bad."), you should be able to back up pretty easily and nothing bad will happen. Just pay more attention in the future.