Looks like bug, but I never see it before. Now I'm not at home so I marked the block as invalid manually. From blockexplorer it looks like the block is ours, but I have to search logs and see what exactly happen. If block is really ours, I'll mark it as valid and distribute the reward for it, of course.
Strange indeed. Thanks for the prompt reply. Sounds about like my luck; I just start on this and within an hour bugs start popping up. ;-)
And I have no idea what JackRabiit is talking about, although it sounds like he thinks that block being marked invalid had something to do with him specifically, as a result of him clicking "Stop" and "Start" quickly in succession?