When the Salmon Queen bashes the skull of one of the sea mutants right after her bikini top is yanked off, she ends up fully topless, but in one shot, her 'beauty pageant' sash and her bikini top strap can be seen on her back.
When Peggy waits for her boyfriend Jerry, and then hears noises and reaches for the knife. She jumps when the kitchen phone rings, and her right arm bra strap drops to just above her elbow. But when Jerry jumps out to scare her the strap is now above her shoulder.
Once the humanoids break through the bottom of the dock, the commotion starts. And the first eight or nine seconds of screaming is "looped" and continually heard throughout the rest of the attack at the carnival.
After the humanoid attacks the couple on the beach, he drags the girl to shore and proceeds to molest her. His head tilts forward and you can see the actor's neck where the monster head separates from the costume.
The goof item below may give away important plot points.
At the end of the film Peggy is giving birth to a humanoid. She has three claw mark scars from her attack earlier in the film, running down her forehead to her chin. But when her belly rises and the humanoid is about to burst out, Peggy's face is totally clear of any scars.