The transition to the pot of boiled cabbage!
After Ralphie does his decoding and exclaims it's just a stupid commercial and opens up the bathroom door, Randy comes in, drops his drawers and lifts the lid to the toilet boil.
Next cut is immediately to the mom taking the lid off the pot of boiled cabbage to stir it. I remember the first time I saw that transition shot and thought naaahhhh! LOL
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And I stepped on the ping pong ball!