I am making an additional and more damning point. That is since each contract WITHIN a block has to be executed to completion before the next (i.e. no parallelization) this also means that the result of the block chain state will be potentially different depending on which order the contracts are ordered in the block. But note that such order is completely arbitrary (because there is no synchrony in distributed systems, i.e. there is no way to prove which transaction arrived first during the block period since each node will have a different ordering due to variance in network propagation).
The becomes an insoluble problem due to chain reorganizations (unless everyone waits for umpteen confirmations before validating anything but then how do we order the blocks...etc...see next paragraph).
Miners decide the order of transactions within a block - unless the block gets orphaned, this remains fixed. Block ordering in PoW chains is
eventual, which implies the deeper a block gets embedded in the chain the higher the probability it won't be reordered. I don't know if etherium scripts start executing after a set number of confirmations, but they almost certainly should do to reduce the probability of the ordering changing.