This happened at a previous employer of mine: the Nintex acquisition of K2. Not long before the acquisition, employees were offered the "opportunity" to purchase their vested options. I didn't invest, but my friend did (so I'm not sure what the details are). They had the power to band together and cause major problems for the deal (I want to say they could outright block it, but I might be hallucinating that). Once they began to organize they were quickly bought out - no details on how much, though.