Revealing mistake: When Calvin falls out of the tree, he lands on the ground on his back. This would at least knock the breath out of him, but he is not hurt at all.
Revealing mistake: When Ted Brookes slides onto the ice pond after falling down the hill, you can see that at first he slides onto an actual ice pond. In the next shot, you can see that the ice has changed and he is now on a fake ice pond when it starts to break.