They can very suddenly crash into another plane because you have looked away for two seconds. It has actually happened (though nowadays, technology has made it easier to avoid these accidents).
Planes are supposed to maintain a 5 mile separation distance. You aren't going to break that down in two seconds. (But head on, with both planes traveling 600 MPH, you can do it in 15 seconds. But both pilots would have to be inattentive for that time.)
They are supposed to, but if flight control doesn't help them do it, pilots have only very little time to react to what they meet. This was demonstrated in the Hughes Airwest collision with an F-4 in 1971.
Since then, air traffic control procedures have been improved to avoid these situations, but nowadays e.g. over the Baltic Sea, Russian military planes are routinely flying with their transponder turned off so that flight control does not know where they are. So, this risk is still there.