Yes, very often 50% of greater you will have a PSAA (posterior superior alveolar artery) within the bony buccal wall as the window is created. Sometimes you can work around it and/or carefully through it with the help of a piezo. If cut you can apply pressure to the hard bony wall to stop bleeding or if large enough plug it up with a Perio probe and lastly you could cauterize it to help stop the bleeding. The sinus cavity has many other areas that provide sufficient arterials blood supply if needed. Having said that, you always want to minimize complications, and carefully work around it.