Since segwit is a soft fork, what would happen if a miner treats some of the segwit outputs as anyone-can-spend scripts (as pre-seg scripts) and spend them in a new block?
SegWit-enabled nodes (ie. non-legacy nodes) will deem the transaction invalid and ignore it.
Legacy nodes (ie. ones that can't validate SegWit transactions as emphasized by nc50lc) will deem the transaction valid and fork off the main chain (assuming there's enough legacy nodes left to propagate this transaction amongst each other). Unless the majority of miners decide to follow this fork it will be orphaned rather quickly though.