Chess Ref,
Happened to me twice last night without penalty.
Ist of two, #4 runs onto the floor and comes to the lower space.
"Who beckoned you onto the floor?"Uh, no one." "We're shootin' two, ya know, but since your'e here you can stay.""Sorry! I got excited." "I know the feeling." [I remember running out onto the gridiron and being called back by the coach to take off my warm-up. 
]
2nd, I'm in the middle of reporting and #3 runs onto the floor from the table. Sent him back. Finished reporting and beckoned him.
Later, he too said, "Sorry!"
mick