Re: [12000 GH/s] Slush's Pool (mining.bitcoin.cz); TX FEES + UserDiff; ASIC tested
2 invalid blocks in a row 
19659 2013-08-18 00:43:31 0:24:25 12918777 3385 0.00626088 252722 25.00000000 invalid
19658 2013-08-18 00:19:06 0:41:44 21756147 5705 0.00652499 252730 25.44870012 invalid
19659 2013-08-18 00:43:31 0:24:25 12918777 3385 0.00626088 252722 25.00000000 invalid
19658 2013-08-18 00:19:06 0:41:44 21756147 5705 0.00652499 252730 25.44870012 invalid
252730 is reported as valid on Blockchain and attributed to Slush. Hopefully Slush will fix this one. The 252722 is defo a weird late one, and not found on Blockchain.
How does and Orphaned block happen?
When more than one solo miner and/or pool find a block at almost the same time each can get added to different instances of the blockchain. When another block is added to one of the instances that one is longer and becomes "The" Chain and the other block is disgarded, orphanned.