It's common to get a job offer that says something like you get 10,000 options on shares. But you don't know if the company has 2M or 20M or however many shares

It's common to get a job offer that says something like you get 10,000 options on shares. But you don't know if the company has 2M or 20M or however many shares; your percentage ownership could be 0.5% or 0.05%. If you're receiving an offer with shares or options, you should ask and be told what the total number of shares are (a cap table would be even better). I have not heard of cases where shares were vested then lost outright, except for perhaps Eduardo Saverin of Facebook. There are other (quite common) cases where the exit is not at a sufficiently high valuation compared to the most recent investment round and preferred shares cash out while common shares are reduced to little or no value.