Sly Stallone showed up at an L.A. courthouse today for jury duty ... and almost immediately got pissed off at the candy machine -- throw the book at that contraption!

Rambo was escorted into the jury room by six -- count them, six -- deputies and an assistant, who filled out all of Sly's paperwork.

Stallone then retreated to the vending machine, apparently in search of a particular type of candy. When it wasn't there, Sly said, "Oh s**t," and then went to another machine and got Skittles.

He was pacing up and down in the jury assembly room. One prospective juror says Sly was "disruptive," speaking loudly as he paced back and forth. At one point, Sly screamed, "Oh f**k." The prospective juror says it looked as if Sly got a Skittle stuck in his teeth.