Revealing mistake: Just before Hermie goes into the drug store to buy the condoms, you can see his breath when he is talking even though it's supposed to be summer time.
Summer of '42 (1971)
Ending / spoiler
Directed by: Robert Mulligan
Starring: Jennifer O'Neill, Gary Grimes, Jerry Houser, Oliver Conant
More mistakes in Summer of '42
Oscy: You use protection. You use a rubber! Haven't you ever heard of a rubber?
Hermie: Yeah, of course, I have.
Oscy: Alright! So, that's what you use. I already have mine. When my brother went into service, he willed it to me. I've been carrying it around with me ever since. Its my lucky charm.
Hermie: Well, how much do you want for it?
Oscy: Hey, its from my brother! It's a family heirloom. You have to get your own. Go to the drug store.