Subdivision Surface and Knife Project. Blender 2.8 I made mesh for deck of guitar. After, i had to make a hole in this mesh. Also i used "Subdivision Surface". I created a circle to use "Knife Project". Made in the mesh a circle then deleted face to make a hole. As u can see, there are some triangles around this hole. If i remove "Subdivision surface" this triangles dissappear.
Ok, the problem you mentioned in the comments is because of these little straggler vertices that aren't connected to the mesh like the others.



So to fix that you can start by deleting this ring of faces:



Next, you create a new circle, place it where you want the hole, select the circle and the rim of the guitar created by the deleting of faces, hit ctrl-E, and select "bridge Edge loops".

And here is the result:



It's definitely not as pretty, but there doesn't seem to be any intersections or anything, and it does get the problem fixed.  Hope that helps.