There is a developing situation in which a bug in the Bitcoin client software is causing a blockchain fork. Those using Bitcoin-Qt v0.8 should not trust the transactions as being non-reversible even if the client shows the transaction having received six or more confirmations.
This latest on the situation may be monitored on the #bitcoin-dev IRC channel.
[The tl:dr on the bug … the v0.8 client software allows a larger transaction that the previous releases did not. As a result v0.7 clients aren’t accepting those blocks.]
[Update: 2013-03-12 02:55 AM UTC. A forum thread by core developer Pieter Wuille has the latest. No further updates will appear in this post.]
[Update: 2013-03-12 02:49 AM UTC. The fork occurred after block 225439. No transactions from block 225439 or prior (corrected) will be affected. Waiting on miners on the v0.7 fork to regain “longest chain” status — expected within a matter of hours.]
[Update: 2013-03-12 02:12 AM UTC. The blockchain built by clients on v0.8 was 6 blocks (perhaps exactly 6 blocks) ahead, thus some transactions that were confirmed will revert to unconfirmed until mined through a v0.7 client.]
[Update: 2013-03-12 02:00 AM UTC. Large mining pools are reverting to use the v0.7 client.]
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