I'm sorry, this is not what I ordered.
or-di-NA-to — four syllables, stress on the third.
When the wrong dish has been brought to your table. Start with 'mi dispiace' to soften the correction.
'Mi dispiace' (I'm sorry) is used here not to apologise for your own action but to soften a complaint — a common Italian politeness strategy. 'Quello che ho ordinato' means 'what I ordered'. This is more effective than a blunt complaint.
Scusi, credo ci sia un errore.
Excuse me, I think there may be a mistake.
Even more diplomatic — 'credo' (I think) signals you are not certain
Ho ordinato le penne, non i rigatoni.
I ordered penne, not rigatoni.
Specific and clear — useful when the mix-up involves a specific item
Questo piatto è per un altro tavolo?
Is this dish for another table?
Very gentle — allows the waiter to save face
Mistakes in Italian restaurants are usually handled graciously. Raising your voice or making a scene is considered very bad form. A polite correction will almost always be fixed immediately and often comes with an apology from the owner.