How did this
turn into this when I rendered it?
Are there two models in the same place?
Try selecting it and moving it, so see if you have a duplicate in the same place. If not, the object might have duplicated parts of the mesh, so try selecting one vertex on the area where it looks dodgy and pressing ‘L’ to select the linked vertices. Move them, see if there is anything in the same place.
Basically the problem tends to happen when you have two faces in exactly the same place.
try remove doubles ,it may solve the problem.
That was it, there was an almost complete copy of that model sitting inside the first. Do you have any ideas on what might’ve caused that and how to avoid it in the future?
I’m not 100% sure, but I think it happens to me when I extrude and cancel but forget to undo one step.
Maybe you hit shift-d by accident. Or as mentioned, if you extruded the area, then right clicked to ‘cancel’, remember it doesn’t cancel the extrude it just doesnt move it, so you end up with an exact copy in the same place.
In the latter case, doing remove doubles will also work.