Was this a real mistake by Douglas?

In the movie there is a scene where Kirk Douglas (Vincent) and Anthony Quinn (Paul) are arguing, and Vincent accidentally calls Paul Theo, and then corrects it. Vincent says "But Theo, I mean Paul...."

It's a funny part and cool that this was left in the movie, but I wonder if this mistake was part of the script, or if Douglas actually made a mistake and corrected himself, but Minelli decided to leave the mistake in the movie because it was funny? Anyone know?