2002 NT7 is an asteroid which may collide with Earth on February 1, 2019. It was first noticed on July 5 of this year, and based on observations made since then scientists believe that it might collide with Earth. This is highly unlikely, however, and future observations are expected to prove that there is no need for alarm. Currently the error in the estimated location of the asteroid on Feb. 1, 2019 is several tens of millions of kilometres. In spite of this, NT7 is still the most "dangerous" space object detected since people started observing them, being the only one to get a positive Palermo scale rating. Scientists say that because of how recently observation of Near-Earth Objects began we can expect many more incidents like this, where an asteroid looks like it might be on collision course but is ruled out by further observations.
In the unlikely case that the asteroid does hit Earth, it will, according to researchers, do so with a velocity of 28 km/sec, which is enough to wipe out an entire continent and cause drastic climate changes.
Source: http://news.bbc.co.uk/2/hi/science/nature/2147879.stm
Update 29/07: analander informs me that the crash in 2019 has officially been ruled out. One possible crash hasn't been ruled out yet, in 2060, but that too is expected to be dismissed following more observations.