In the 1920s and early 1930s this worrying pronouncement could be found on bottles of grape concentrate sold in the United States.
Due to Prohibition, grape growers could no longer produce wine, but it was still legal to produce non-alcoholic grape products; and it was discovered that, under certain conditions, grape concentrate could accidentally ferment into wine. Of course it was necessary to warn the public of this malign possibility, and thus the large, emboldened label was added to each bottle.
Just to be on the safe side, the bottles also featured step-by-step details of how this unwanted transformation might occur; this labelling was within the law, and helpful to the consumer who wished not to inadvertently produce an illegal substance in his own home.
Grape concentrate became a surprisingly successful product in the 1920s, so much so that "wine consumption actually trebled, and [between 1925 and 1939] California's vineyards expanded from less than 100,000 acres before Prohibition to almost 700,000 acres afterwards." (Quote from Bill Bryson's Made in America.)
Bizarrely, however, the end of Prohibition coincided with a enormous decrease in the market for grape concentrate, a healthy and nutritious drink for which the American public seems to have had an all-too-brief fondness.
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