When is turbulence most dangerous to an aircraft?
Wake turbulence can affect aircraft in the most critical phases of flight such as landing approach.

For example, consider this AOPA report


  On June 12, 2006, while on visual approach at Kansas City International Airport in Kansas City, Mo., the pilot of a Piper Saratoga crossed below the flight path of a Boeing 737 that was landing ahead on a parallel runway. The Saratoga encountered wake turbulence so violent that it tore apart the aircraft in flight. The pilot and his passenger were killed.


The effects are most severe for small aircraft entering the wake of a large aircraft but large aircraft can be affected too.

Skybrary lists several examples of incidents involving wake turbulance.