Barack Obama -- Cross The Imaginary Line at Your Peril!!!

A jolted jogger learned a scary lesson Saturday -- cross the imaginary line in the sand near President Obama's Hawaii retreat ... and the Secret Service will have your ass!

The jogger was happily barreling along on Kailua beach ... near Honolulu. Notice what looks like a palm frond plunked in the sand ... near the home where the Obamas are staying.

There's no sign ... no real way to know that's the line in the sand you just can't cross.

After the agents caught up with the jogger, he was sternly ordered away.