An oldie, but a goodie...
Overheard while flying freight near Frankfurt.

ATC: F1243, desend and maintain 12-thousand.

F1234: 12-thousand, F1234.

ATC: F1234, can you make it to 12 in one minute?

(short pause)

F1234: Negative. The captain requests I inform you we're going as fast as this Fokker will go.

(silence)

ATC: ...Right. Lufthansa 456, turn right heading 330, please.