Continuity mistake: When Finian and Og are talking in the forest, Og is kneeling on a log. He leans over and falls into the river. You can see him completely submerged. He stands up and continues his conversation with Finian while standing in the river and his hair is now completely dry.
Continuity mistake: When Finian goes into the woods to bury the crock of gold, there is an overhead shot looking down on him while he is digging the hole. It shows quite a lot of dirt between the pile of dirt he has heaped up and the hole, enough to cover up the grass immediately around the hole on that side. But in the shot right after he places the crock in the ground, there is no dirt on the grass around the hole.
Finian McLonergan: You're an imposter! You can't be leprechaun you're too tall.
Og: I know, and I'm getting taller.