On January 21, 1971, Pluto crossed Neptune’s orbit making Pluto the 8th planet from the Sun. On February 11, 1999, Pluto crossed back over Neptune’s orbit to regain its position as the 9th planet from the Sun. It will stay this way for over 200 years. A visual image of the two planets’ orbits can be seen here.
Despite the “crossing of orbits” between the two planets, they will never collide with each other. This is due to two reasons. The first reason is due to their orbits being in resonance. This means that for every two orbits Neptune makes around the Sun, Pluto goes around three times. When Pluto is close to the crossing point, Neptune is somewhere entirely different on its orbit.
The second reason for no such collision is due to the fact that Pluto has a higher orbital plane than Neptune. According to NASA, Pluto will actually never come closer than two billion kilometers from Neptune.