Philae has successfully landed on comet Comet 67P/Churyumov-Gerasimenko, but its landing harpoons didn't fire. Presumably, its ice screws did, though. The comet is between 3-4 km wide, so it has little surface gravity, and the the lander could detach if it tries to do anything that pushes itself towards the comet too hard (think of how you bounce in the opposite direction when you push hard against something).

If it detaches, there's no way for the orbiter to bring it back. This was a one-way trip. But it should already have interesting data.

ETA: Apparently, Philae landed and started sending back information on the surface it encountered, then "bounced" and relanded.

ETA ETA: then bounced and relanded AGAIN.

ETA ETA ETA: then it landed on its side against a "rock" face. It isn't anchored, but its down, and otherwise functioning as expected.