If you look at this White Castle ad closely, you'll see that the "limited time" language doesn't specify an expiration year. But don't be a jerk and try to cash in the coupon.
 
No need to strain your eyes, dear reader: Click on the image above for a larger version of this ad, which appeared in the Minneapolis Star.

 

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Jan. 23, 1915: 'Meanest boy' cuts off girl's hair

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Aug. 2, 1896: An ad man, droll and dapper