It's an ongoing thing between a friend and me. We sow seeds of doubt in each other (by sending creepy messages or whispering 'wake up' out of the side of the mouth without changing the facial expression or viewing direction), joke about it and are both left riddled with thoughts about the possible unreality of everything. :D
Have fun!
RE: "Repeatedly run into the same stranger and tell them to 'wake up!' and that they are still in a coma..."