I've tried to make a model according to the lesson https://www.youtube.com/watch?v=EztmPOhnWwA. After applying the subdivision surface modifier, I faced an issue when one of the "tongues" displayed incorrectly. I suspected that the problem might be caused by duplicated vertices but "Merge -> By distance" didn't help. Then, I've turned on displaying of normals for vertices, and deleted a part of the "tongue's" vertices. After I did it, three normals appeared in a place that was empty (it seems the "tongue" was in this place until I moved it). The vertices which the normals may belong to are not visible. When I'm trying to select them with the box selection tool in wireframe mode, and delete, nothing happens. When I select all the vertices with hotkey "A" and delete them, these three normals remained on the scene while the other vertices were deleted. I suspect that it might be a bug in Blender but since I'm a novice, I'm not sure. I filmed a video with the issue demonstration: https://youtu.be/5QpX0yr1_OY. Blender version - 2.92.0. If someone is interested, link to the .blend file: https://drive.google.com/file/d/12sMXc79xpEDRgyUej0e3-2WO93rTlFcx/view?usp=sharing
It looks like you accidentally hid those faces/vertices.. If you open that model again, go into 'Edit Mode' then press 'ALT+H' you'll see that those vertices/faces were, maybe by mistake, hidden. 'H' hides anything that you have selected. and 'ALT+H' unhides them. If you selected those faces, verts or edges and accidentally pressed 'H' then you probably hid them so they'd remain hidden until you press 'ALT+H' to unhide them.