It's an urban legend that's floated around in various forms: in some it's an ice cream parlor rather than a store and they pack the vanilla faster, in some it's only the vanilla that gets hand-packed so it takes longer, it's pistachio that takes longer and triggers the problem.

Snopes covered this one and they cite to an urban legends book from 1989: "Curses, Broiled Again" by Jan Harold Brunvand. Brunvand prints the "vanilla takes longer" version and reports also having a "pistachio takes longer" version printed in a magazine in 1978, which itself referred to another magazine as its source. In the book's version it's a Texas car dealer who's looking into the problem. The same author's later book from 1999 covers the story again and includes versions dating back further, plus a 1992 version which is this one where it's Pontiac and the problem is the vanilla being in a separate case at the front of the store.

Maybe there's some kernel of a true story in there, but if so it's probably a pretty small kernel. Anyway it doesn't matter much: it's just a fun story that teaches a little lesson so people like to share it around.