Some startups take your equity off you if you leave.
http://stockoptioncounsel.com/blog/standards-ownership-canth...
I never saw anything like this when I worked at an early stage startup, and it's obviously something an employee who reads their employment agreement would ask to be taken out. I'd never work at a company with that clause.
On the other hand, this can lead to disaffected employees who are only staying with the job because they are waiting for their options to mature.
Would anyone join that company going forward? Would they be able to raise money from investors? Would their attorney drop them?