Quote from: ArsenShnurkov on March 07, 2011, 08:18:26 PM

Quote from: gavinandresen on March 07, 2011, 07:57:34 PM

Old clients will refuse to relay sendmany transactions

Using this way anyone with new client can doublespend money - send once to client with new version, then send second time to the client with old version.
If someone confirm the second transaction (which is theoretically possible, while not everybody miner update their software), the money will be doublespent.

I think you misunderstand the latitude miners are given.

A block may only belong to the 'best' chain if its transaction inputs may be connected and verified.

Thus, anyone who creates a block may create a non-standard transaction, but that transaction -- once it makes it into a block -- must still be verified by the same basic checks that guard against incorrect signatures, double-spending, etc.