When the Wiggles run around backstage, after the teacher blows the whistle, Greg is wearing a wedding band in a few shots.
When Captain Feathersword is talking to Wally, one of the pirates in the background is mouthing Captain Featherswords words.
The camera wobbles just before Jeff comes out of his room wearing Murray's red shirt and says "Nope, no Dorothy".
During "Rockin' and a Rollin' Sea", the pool moves a bit between Greg and Murray's jump into the water. This is likely the fault of it being moved accidentally after those particular takes.
A rope is seen pulling the Big Red Car when The Wiggles leave the Sandlot. This is cropped out of frame for the movie itself in widescreen as it was made in an open matte ratio and shown in theatres in widescreen, so it isn't meant to be shown. In the 1998 VHS, they applied a digital zoom on the picture right before it happens so it wouldn't be visible then they zoom out the picture again during the following shot while the camera is panning back so it's not as noticeable.
The 2003 "Magical Adventure" version uses different picture cropping, so the zoom isn't there, but the rope can be seen briefly as the Big Red Car arrives at the Sandlot and when it leaves.
During "Rock-A-Bye Your Bear", Murray is playing the electric guitar but the music presented features an acoustic guitar.
In "Quack Quack", when Murray wakes Jeff up, his seat belt falls down but he fixes it afterwards.
When Captain Feathersword says "I will be seeing you tonight." his right arm bumps the actress next to him.
When Murray was getting ready for the concert, the guitar strap is placed on the correct way. But during the concert performance, it was placed on backwards.