It’s true! Don’t ever try to ask for ketchup on pasta! Never! I say NEVER do that! You’ll obtain pissed off reactions!!! Believe me!
My foreign colleague collegue wasn’t allowed to put ketchup on pasta when Italians were present! LOL!!
Don’t even try to ask for cheese on fish! It’s real blasphemy in Italy.
If you ask the waiter some cheese to be put on pasta with clams, he’ll probably pretend he didn’t hear you!
If he brings you ketchup for pasta or cheese for fish, probably the restaurant has a touristic target!