42 floors has an interesting mandatory 2 weeks vacation when you start working there. [0]
Paperplanes has a mandatory minimum vacation policy on top of their unlimited vacation policy, in order to prevent employees from feeling like they can never take a break since there's always more to do (a common issue with unlimited vacation policies).
As for my own startup, we're still super early stage so its "unlimited" but I don't foresee us being able to take a break anytime soon. :/