Along with the Spanish Inquisition, no one expects Jeffrey Dean Morgan.
Sometimes written as Surprise!JDM, Surprise JDM is said to occur when Jeffrey Dean Morgan appears in a RPF story and has sex with either Jensen or Jared or both , without warning through the pairings listed in the header of the fic.
People who are heavily invested in the OTP can find this distressing.
Warning for Surprise JDM is advised.
JDM's appearance at Salute to Supernatural Las Vegas 2015 Saturday Night Special Concert, certainly counts as an example of the finest order of Surprise JDM.
In the case of JDM's return to Supernatural in the 300th episode 14.13 Lebanon, it was good to see that the network did, in the form of much publicity, warn for Surprise!JDM. Just as well as a shock of this magnitude may have killed many in fandom.