It’s been a headline-grabbing year for Internet M&A, with large-cap companies such as Apple, Facebook and Amazon.com making their biggest acquisitions in a long time, or ever. There have also been some multibillion-dollar consolidations (Zillow-Trulia, Priceline-OpenTable) as well as several high-profile spin-off announcements that could portend more deal activity next year.

Overall, the value of media, information, marketing and tech mergers rose to $94 billion so far this year from $45 billion during the same period last year, according to Jordan Edmiston Group.

But dealmakers should heed some cautionary tales and remember the long history of big M&A flops that once seemed like high flyers. Many recent deals—including those for so-called “unicorns” worth more than a billion dollars—will fade into oblivion just as deals often have in the past. It’s hard to pinpoint just how much money was flushed down the drain.