The initial external shot of the airborne Boeing 747 is a full-length version of the plane. Upon landing, it is clearly the much-shorter "SP" version of a TWA 747.
When the big Kahuna is headed to the surf-off , the front end of his board is round. During the surf-off , the front is pointed. After the surf-off , it is round again.
During the Big Beach Bonfire Party, Annette starts out wearing an orange sweater over a sparkly white top and pants. Minutes later, she's wearing a bright pink wetsuit with a polka-dot skirt while she surfs with Troy. She goes off to find Connie in the very next scene and she's back to wearing the original outfit.
During a long dolly shot of the singing cast marching down the beach, wheel tracks in the sand from the camera car are visible.