As a young and naive engineer I learned of this fact the day the first startup I worked for was acquired. First I read the big number that was to be paid for the company, was ecstatic, and immediately starting doing "x-million times half a percent" in my head followed by a sinking feeling as I read the clause stating that common stock holders would get $0.
In retrospect it sounds obvious that if the company sells for less than the money the investors put in, your x percent is worth nothing. But it's easy to get carried away thinking you actually own a percent of the company, and that a sale means a pay day for you. Don't let the first word of acquisition get you too excited, the come down sucks.