I doubt 270 mph has been exceeded but it is possible depending on the aircraft. 

The worst case scenario would be a max weight takeoff followed by an immediate flap less landing.  

The B747-800 has a max takeoff weight over 900,000 lbs. If one had to had to do a flap-less emergency landing shortly after a max weight takeoff (and unable to dump fuel), it would use Vref 30+40 (about 226 knots) for an approach speed, and that would be about 260 mph.

I don't have access to all the B747 speeds, but this chart does give an idea of the speeds needed for a flaps up landing. 

Source: B-747-400 Flight Crew Training manual
[![enter image description here][1]][1]



  [1]: https://i.sstatic.net/ayD6C.jpg
  [2]: https://i.sstatic.net/1uElP.jpg