258617 is claimed by BTCGuild
258589 is claimed by EclipseMC

What happened is: We found a block nearly at the same time as someone else, and they were lucky enough to have the next block found based upon their block instead of ours.
So our blocks are orphaned (not part of the main chain) and thus invalid.
Its bad luck, but happens from time to time, because the info that some block was found does not appear at instant speed at all nodes.
The time needed for the block to propagate through the network leaves a small time window for another block with same block # to be found.