EDIT: I'm being downvoted, but I've been increasingly hearing a shady Silicon Valley practice where, upon successful acquisition, the board will vote to emit a large number of new shares (think 5-10X the total pool), which will be redistributed just among execs and investors. So, if you are an employee who held on to your 0.1% (which, on 1B, might be worth 1M), you might find out that after the acquisition you are going to be diluted maybe to 0.01%. And this is after all the other "healthy" dilutions that have happened to the company over the years, as part of their financing rounds.