It's only happened a few times, but my spidey-senses are honed to it now: When your clinician for the weekend calls while you're teaching, leaves a voicemail, and then follows it up with a text message saying: "Call me ASAP," it's not a good thing.
Poor Michael, Mr. Tough Guy, was felled by the Plague (or the flu, or a sinus infection, or something) and couldn't make it for the weekend.
Me, on the outside: Cool as a cucumber. "No problem, you poor thing! Feel better soon."
Me, on the inside: FREAK THE HECK OUT.