sighted writes "The twin lunar Gravity Recovery and Interior Laboratory (GRAIL) spacecraft lifted off from Cape Canaveral this morning. GRAIL-A is scheduled to reach the moon on New Year's Eve 2011, while GRAIL-B will arrive New Year's Day 2012. The two solar-powered spacecraft will fly in tandem orbits around the moon to measure its gravity field. Lunar explorers hope the mission will answer longstanding questions about the moon 'from crust to core.'" Link to Original Source
If it happens once, it's a bug. If it happens twice, it's a feature.
If it happens more than twice, it's a design philosophy.