1. Do transactions ever time out if not accepted into the block chain?
e.g. if I create a transaction today, and it happens to be invalid, is it possible it will unexpectedly get accepted, e.g. with a later release of the bitcoin block chain code that has policy changes, maybe in x years time? (assuming transaction this would have to be accepted by miner *and* the block chain code)
Or for another example, maybe one less far fetched, consider a non-standard transaction, that isn't included in the block chain any time soon. Is it possible that a miner could unexpectedly process this transaction at any undetermined point in time in the future of bitcoin? Or will the transaction eventually expire?