Let's say that the employee has 0.4% (after dilution) of BuzzFlop with a 4-year vesting cycle. After 2.5 years, BuzzFlop is bought by Hooli for $100M in Hooli stock. The employee doesn't get $250k in walk-away cash, but $250k in Hooli stock, subject itself to a vesting schedule (and possibly a "refresher"). If that employee gets cliffed (which can happen in a merger) then none of that stock ever vests.