Many, many ways.

First of all, "high" is relative. The exit may be high, but still not cover the funding, so all the proceeds go to the funding entities (the VCs) and maybe a few top executives / founders. That happens in countless startups that sell for $50m after raising $30-40m. Even "senior" employees get basically nothing.

Another common issue is dilution. Your vested shares can and will be severely diluted. Again, the VCs and maybe the founders are the only ones with any control or protection over this.

A lot of other interesting things can happen at or around that all important payday. You may want to read about how things like IPO and other liquidity events are actually handled by law. Surprisingly few startup employees do, and this stuff is far from trivial - there's a reason why Goldman et al pay the best and brightest to figure this stuff out.

All of this is doubly true if the founders / execs are untrustworthy / morally indifferent and actually strive to dilute or otherwise deprive you.

Our very first investment agreement (3i as main investor with a local VC doing the legwork) had a table of equations defining various things that would effect how much founders and option holders would get in the event of an exit. Working out what these actually meant was non-trivial and we argued strongly against them - no luck.

Ironically when we went public we made a chunk of money from the very ratchet agreements we had argued so vigorously against (combined with help from the taxman).