I am an ATC working a large en-route sector which has about 10 frequencies, all re-transmitting.
Sometimes we have about 25 aircraft in the airspace.....5-6 Virgin....5-6 Qantas...and multiples of most airlines.
It never ceases to amaze me just how similar many of the flight numbers are.
In the past fortnight I would have had at least 3 wrong aircraft responding to another aircrafts instruction. Thank god never for a level change....usually just a frequency transfer. And some of these guys switch INSTANTLY, so ya don't get a chance to correct it.