I agree with you Turnbull. It happened too often throughout the film for it to be coincidental. I have no proof and this is just speculation but I wonder if he was paying homage to the original Scarface where an X appeared somewhere on screen before someone got killed.

https://www.nytimes.com/1971/08/15/...t-of-a-home-movie-the-making-of-the.html

great article about filming the scene where Vito got shot. A couple of Mafia men were on the set but they weren't too impressed with Coppola's fanatical attention to detail. Here is a snippet.

Back on Mott Street, two Mafiosi assigned to observe the movie production were unaware of his arrival. For hours, they had been watching Brando getting shot. They had had innumerable cups of coffee and had adjusted their open-throat, hand-ironed shirts so often that their collars had begun to wilt. Neither of them had been impressed when they heard Brando was to play the godfather, so they watched his performance critically. They volunteered to grips, cameramen and extras that they would have preferred Ernest Borgnine or Anthony Quinn.

''A man of that stature,'' one of them said, pointing to Brando, ''would never wear a hat like that. They never pinched them in the front like that. Italian block, that's the way they wore them, Italian block.''

They did not like Brando's wearing his belt below his trouser loops either.

''He makes the old man look like an iceman. That's not right. A man like that had style. He should have a diamond belt buckle. They hall had diamond belt buckles. And a diamond ring and tie clasp. Those old bosses loved diamonds. They all wore them. Brando makes the guy look like an iceman.''

Check it out. Interesting read.