The Dutch newspaper de Volkskrant had a lovely article a few days ago about the ten biggest scientific bloopers of 2007 (in Dutch). There’s a good astronomy one at #9:
A minor panic arose amongst astronomers in early November, when Richard Kowalski discovered a planetoid to be on an almost direct course for Earth. He estimated it to pass within 5700 km (3500 mi) of the Earth’s surface on November 13th. Some months earlier, the planetoid, designated 2007 VN84, had almost hit Mars.
Reading the announcement, Denis Denisenko of the Russian institute for space research was immediately reminded of the Rosetta spacecraft. Rosetta had done a fly-by of Mars on February 25th and would do a fly-by of Earth on November 13th to gain speed to reach its ultimate goal, the comet 67P/Churyumov-Gerasimenko. A brief discussion on the Minor Planet Mailing List confirmed that Rosetta and “planetoid” 2007 VN84 were one and the same object. The Minor Planet Center, which tracks planetoids near Earth, quickly erased 2007 VN84 from their records.