stolen from http://www.pprune.org/tech-log/423988-concorde-question-3.ht...
Ancient tale.
There's this SR-71 Blackbird stooging around Cuba on a top-secret mission, at FL500+ and Mach 2+.... when they get a call requesting them to change heading "because of traffic at your altitude". Traffic at THEIR altitude ?? Anyway, they comply, and shortly, yes, there's an Air France Concorde out of Caracas (Air France flew there in the early days) slowly sailing across their flight path.
Just imagine... two guys in bonedomes and full pressure suits, in a cramped cockpit, watching something like a hundred people in shirt sleeves or summer dresses, sipping their champagne and maybe just starting on their smoked salmon hors d'oeuvres, flying at their altitude and nearly their speed....