@Catbites.
My 2 cents on your query.
When i attempt questions, i mark them on the question paper (by circling the option or putting a small tick next to it). when i come back to those questions, sometimes I change my mind and select a different option, mark it on the OMR (Usually at the end and in a hurry and perhaps forget to change the answer on the question paper) . Since both of the options were pretty close (in my mind), when I go back home and cross-check it with the answer keys, at times I am pretty confused about which option I marked on the OMR...The one tht I have marked on the Question Paper or the other one...
OMR sheet adds to the fun..while marking 140 Odd questions, even though you are clear in your mind about the answer, you might have marked a different option on the OMR and while checking with the answer key, you reckon your answer to be correct, but OMR reflects otherwise.
And...On OMR... Incomplete/incorrect shading (By Pen in SNAP!) makes matter worse!
:)