You absolutely can spend change-in-flight – the default client even allowed it, last I checked. It's just the risk of third-party mutation, as under the curren

You absolutely can spend change-in-flight – the default client even allowed it, last I checked. It's just the risk of third-party mutation, as under the current mischief-making malleability attack, plus the compounded risk created by chains of unconfirmed-transactions, that makes this a problem. Long before the current attacks, I had my own outgoing payments stuck when they were inadvertently arranged as a dependent chain, and the first transaction never confirmed. But once you (or the people writing your software) know of such risk, mitigating workarounds are easy and automatable: You arrange for receipts of large amounts to be broken into smaller units, capping the amount of change that might be 'in flight'. (If necessary, you do this break-up yourself upon receipt, knowing there's a period of up to an hour before that completes.) Or more generally, you keep small working amounts of mixed-size coins ready for short-term use, simply not expecting to spend a large new receipt instantly. (No one who is living 'hand to mouth' should be relying on Bitcoin for all their life's necessities, anyway. It's still beta!) When the third-party mutated-transactions risk is contained – within a week or two, all major miners will probably blockade tampered transactions – this risk of stuck funds will evaporate... but the lessons and new software practices for handling corner cases will persist, ready for future similar situations. It's interesting that patio11 is using receipt-of-salary as an example. Until your bank knows you, classic paychecks can be held for a day or more before full funds are available. Until clearing-practices were changed in the US a few years ago, large paychecks often had most of their funds held as unavailable for 7-10 days – even for checks between large American banks in the same city. This situation persisted for decades. Further, the fraud of "check-kiting", which is vaguely like the interplay-of-timing-bugs that has tricked some bitcoin-payout systems, was possible in the traditional banking system, on and off, for decades. Compared to that, are Bitcoin's temporary problems, perhaps involving an hour's holdup, and likely fixable within a week via rapid software iteration, that big of a deal? When patio11 makes people aware of the transient gotchas and rough-edges, so they can adjust their near-term expectations, that's a public service. But if these things are portrayed as inherent and insurmountable weaknesses that doom the system? That viewpoint is going to look like 1995 Clifford Stoll.