Mergers and acquisitions play a key role in the tech industry, fueling start-ups and supplying existing firms with innovative new technology. Unfortunately, mergers often go awry, and they sometimes cost shareholders billions.

Below are five of the largest and most disastrous mergers in tech history. Although they were entered into with the best of intentions, they failed to generate the sorts of synergies management was hoping for. In some cases, they involved equals attempting to create a more significant market player. In others, a larger company purchased a smaller one for its technology. The one thing they have in common is that they all failed.