When Mr. Peters is shooting at Mannix on the target range, he blows large holes in the targets with his shotgun. The range between Mr. Peters and the targets is too great for a shotgun to cause such damage; the pellet dispersal would be too wide, and the pellets might not even penetrate the targets.
Dr Baker from LHOTP or the goon du jour here, lightly taps Mannix between his shoulder blades with the tip of his gun, and the good detective ridiculously falls asleep, instantly at that.
During the big fight scene on the water, the bad guy falls backwards and his fly is unzipped. Next frame he's back up again and this was corrected.
When Mannix turns into the driveway of Celia's apartment building, there are 2 people staring out of the first floor apt (top left of screen) window watching the filming action.