Goofs

The first murder is committed with a prop gun, specifically a semi-automatic pistol modified to fire blanks. Such guns have bore restricters, weakened springs and ground-off locking lugs, all necessary to operate with the weak chamber pressure and recoil impetus of blanks; they invariably blow up or fly apart if fired with live ammunition.

Right after Jackson Sidemark and Winifred Glover talk, the director tells everyone to get ready on the set. The actor trips on the hay bale as he's climbing up the fake hill.

In the bar scene, piano music is heard but the musician is playing an old pump organ.

After Barry is found shot dead, there is a quick set of close-ups of some of the on-set witnesses. Included is Dick Clark's character, Lief Early. Yet later during court testimony, Clark claims he was not on the set at that time, but none of the other crew or cast seems to recall seeing him there...

Boom mic visible in the opening scene after the director calls cut.

Leif Early testified that he was not at the scene of the first murder. Right after the murder is committed, there is a shot of Early at the murder scene, standing in the foreground with several other people close by. Most of them would probably remember seeing him there, and so Early would realize he could not get away with lying about it on the witness stand.

Perry Mason dismisses Leif Early's rant against Hollywood on the witness stand as being irrelevant, but then allows the prosecution to call forth a parade of witnesses to testify about Winfred Glover's past bad behavior in matters completely unrelated to the case, without offering a single objection.