The morning after Harold places the pipe bomb on the car, he is shown lying in bed on his back (from a top down camera angle). The phone rings, and the camera angle switches to a side shot, and he is now lying on his right side. No time had passed to allow him to switch positions.
When Columbo originally watches the tape, there is no cut between the gardener entering the car and the explosion. When he shows the tape to Dolores, the tape cuts before the explosion.
The final tape showing Harold 'wincing' has a cut in it, from just the gardener getting into the car, to a shot including Harold with the car in background. If it was one camera filming in real time, there would have been no cut.
When Columbo is examining the alarm clock after Harold's death, he states that the alarm wasn't set for 8:00 a.m. but rather it was set for 8:00 p.m. In fact the clock was an analog clock that had no am/pm. Such a clock would actually go off at both times. That said, Harold could have intended to awake at 8:00 p.m. if that was the next time the clock reached 8:00 o'clock after setting the alarm.
When Columbo shows Dolores the video playback of the Rolls Royce exploding, a crew member can be seen in front of the car kicking a piece of bodywork (presumably a wing or bonnet section) across the shot.
Tyne Daly's "Southern accent" comes and goes throughout the episode.