Well, if one side is setting up an ambush then they are already aware of the other party, so there would be no chance for that party to surprise the ambushers.
"I, Satampra Zeiros of Uzuldaroum, shall write with my left hand, since I have no longer any other, the tale of everything that befell Tirouv Ompallios and myself in the shrine of the god Tsathoggua..."