If I verbally accept an offer that was never put in writing, I feel like one of three things could happen:
1: there is something materially and surprisingly bad in the contract (equity or cash isn't as described)
2: there is something that is arguably not material, but which is surprisingly bad in the contract (something to do with vesting schedule, clawback of a 'signing bonus')
3: everything is as-expected.
In case 1, it's clear that they have materially changed the terms, and I am no longer bound by my verbal acceptance (which was of a different offer).
In case 2, they can argue that what was changed isn't material, and so I'm still bound. But if they say that, I would reply that if it's not material, then they should have no problem changing it back to the originally-discussed state. If they don't, I walk. It's not like they can force you to work for them, and they would presumably not try to ruin your rep over it because you could probably do more damage to them (by posting on HN, for example).
In case 3, obviously everything is fine!