If you have facility of CBCT; you may get proper dimensions of bone and prevent implant in sinus in most cases. However in some cases if bone resorbs between the sinus and the implant; the implant may land up in the sinus or to get a proper bi cortical support you deliberately go for few mm in the sinus. In most cases there is no problem. If there is any issue of sinus infection repeatedly than you can think of removing the implant from sinus by the window technique. Partially embedded implants in the sinus may be unscrewed and a new shorter implant with 0.5 mm higher diameter can be reinserted. Getting the right torque is important.
Now the question arises what if you do not get the right torque and the implant is not stable. The best way to try higher diameter of implants in increments of 0.5 mm and get the right torque. If you feel you do not have enough bone diameter to support; you may expand the bone and condense the bone or you may use a plate usually used by Orthopaedic surgeons or maxillofacial surgeons with multiple holes. You may put the plate such that the end two holes are screwed in the bone and your implant can be in the center hole. This will give primary stability to your implant. If you require and feel necessary you may do a bone graft or sinus lifting before you screw the plate and insert the implant. Another way is you may put implant in slant direction such that it is in the bone as done in all on 4 technique and the sinus can be prevented.
In zygomatic implants it is very common to see implant going thru the sinus. This is perfectly OK and usually does not require intervention.