While filming the “parking” scene with Marty and young Lorraine in the car, the production crew decided to play a practical joke at Michael J. Fox’s expense. The scene called for Fox to drink from a prop liquor bottle filled with water and do a spit take when he sees Lorraine with a cigarette. For a specific take however, the prop liquor bottle was switched for one which contained real alcohol inside. Fox, unaware of this, performed the scene and drank from the bottle, only to discover the switch after-the-fact. The full gag is featured on the “Outtakes” section of the DVD.
(Source: filmtrivia)
This was hilarious!!!! Watch the outtakes on YouTube, it’s so funny!!