Mine was with headphones, late at night, around 3am, trying to finish some work... had been listening to some music, forget the track/artist but there sneaky mixing artist had made the last few min of one track very quiet, forcing me to turn up the volume to full... then when the next song came in it was a vocal sample of a guy saying "y'all ready!" (only found this out after re-listening the next day... at the time it was just a blood curdling scream), only on the right speaker, and full blast with a bit of reverb to make it sound like it was behind you.... needless to say the kitchen was to the right of me and I was 100% sure there was a 7 foot tall 300lbs bandit ready to bash my head in with a baseball bat. I physically jumped out of my chair and clothes-lined myself on my headphones... and as Michael said, the adrenaline was instant and copious, my heart still hasn't recovered...
The really stupid thing is though... that it happened again!

sorry for going off topic, just wanted to thank Michael for reminding me of that crazy night.