Market cannibalism is defined as the negative impact a company's new product has on the sales performance of existing products. This is best illustrated by the "Cola Wars" -
the marketing fight between Pepsi (NYSE:PEP) and Coca-Cola (NYSE:COKE), which lasted most of the 1970s and 1980s. The soft drink rivalry pushed Coca-Cola Co. to make one of the most famous marketing blunders in financial history. In the process of creating Diet Coke, the company's chemists discovered a new formulation for Coke. The new concoction was sweeter and smoother than the century-old formula upon which Coke had been built. In fact, it was similar to Pepsi - the drink that was eating away at Coke's domestic market share.
On April 23, 1985, Coca-Cola Co. announced that New Coke was on its way. Because of a strong preference for New Coke in consumer taste tests, Coca-Cola decided to pull the old Coke formula from the shelves. Essentially, the company was throwing away a century of branding by favoring the new, relatively unknown formula over the one that consumers had grown up with. For Coca-Cola executives, this made sense. Much like with software companies that pull old versions from the shelf when a new one is released, they didn't want their old product line to keep consumers from buying their new one. Unfortunately, this bold move backfired horribly.
Consumers rebelled and flooded Coca-Cola with angry letters and phone calls. Coke's stock and market share took multiple hits and Pepsi even proclaimed victory in the Cola Wars now that Coca-Cola had copied its taste. The influx of complaints led to a "We've heard you" marketing reverse. On July 11, 1985, mere months after its sudden exit, the old formula was re-introduced with "Classic" added to the title - probably better than "Old Coke". Coca-Cola Classic quickly ate up the sales of New Coke in a textbook case of market cannibalization, but the company's stock did recover for the most part. The marketing blunder may not have been as much of a disaster as it appears. The controversy and media attention attracted some fence-sitting consumers back to the Coca-Cola brand.
Nevertheless, the saga of New Coke turned off many investors and resulted in Coca-Cola becoming an undervalued wallflower that nobody wanted to touch. Due to the strong international presence of Coke, however, investing sage Warren Buffet started buying significant amounts of Coca-Cola stock in the late '80s, which proved to be one of his most profitable buys. Despite its flirtation with a branding disaster and market cannibalization, Coke remains one of the world's strongest brands and a stalwart company to boot.
(For more on this topic, read Warren Buffet: The Road to Riches.)
This question was answered by Andrew Beattie.