This one will be talked about for years:
"Facebook’s deal to buy Instagram for $1 billion stunned Wall Street and Silicon Valley when it was announced in April. But executives at Twitter had an additional reason to be surprised. Instagram’s founders “held several meetings as late as March with top Twitter executives,” The New York Times’s Nick Bilton reports. “The sides had verbally agreed weeks earlier on a price for Instagram of $525 million in cash and Twitter shares,”"
Interestingly the deal closed for about $735m down from the $1billion that was based on pre-IPO valuations.