Limit one coupon per purchase

[snopes]

Comment: I heard a story in the early 1970s about the origin of the
wording on coupons that says "limit one coupon per purchase". My uncle
told me that some years back there was a coupon in a newspaper for $100
toward a Cadillac. An enterprising woman gathered up enough coupons to buy
a Cadillac outright. The dealership she went to, after much argument, was
forced to give her the car because nowhere did it say "limit one coupon
per purchase". I seem to recall my uncle saying this episode took place in
Chicago.

I'd be interested to know if this is a complete fabrication, or if it's
true but with different details.

[lord_feldon]

Is this the same principle that "forces" casinos and banks to give you the millions of dollars when an error is made?

[KirkMcD]

Don't know about the story, but sometimes a coupon can be used for multiple purchases of the same item.
The wording in the OP is to prevent that.