I agree with much of FF's comments I would add that with some students complacency sets in quite quickly and items are over looked.
Perhaps on the human factor side, I think some students think because they have got an instructor sat next to them that any oversight in checking will be corrected by him/her before a problem manifests.
Another strange one is, the student seems to rush through checking particularly pre-start, because they are concerned by the fact that you are bored of seeing this done numerous times before!