I'd call it “accidental” when, for example, a selfmade TV antenna with amplifier starts to oscillate and thereby blocks GPS reception in a harbor:

http://gpsworld.com/the-hunt-rfi/

But the jamming device owned by the grossly negligent truck driver performed even better than designed (if one would call these brute-force wideband jammers “designed.”). And the fine imposed on the guy for operating those deliberately is well earned.

{edit: spelling, wording}