I know I've seen this same bug reported in another thread, but darned if I can find it.
The problem is capturing an ally's planet. This occurred when both an ally's and an enemy's planet were in the same sector. The enemy planet was slagged and the ally planet was captured.
I sent the bug report in to Malfador just hours ago and received a reply from Aaron already! Does this guy ever take a day off?
The good news: he's going to try to have it fixed for the next patch.