For me it's 1991's Shattered. Toward the end of the movie, Bob Hoskins gets thrown in some water and disappears, presumed drowned. Later on he pops up, alive, holding his asthma inhaler. "I've got my own tank!"
For most of the 90s I then assumed that asthma inhalers worked through concentrated oxygen or something else that could work as a mini-scuba tank.
What other misinformation have people accidentally swallowed?