It is the responsibility of the instructors or colleges/department representative to pickup the OMR sheets within 72 hours of completion of processing.
The scanning system can only indicate whether the scanning hardware determines a bubble is filled in above a threshold. Similarly, the scanning hardware must determine whether a bubble has been erased completely enough to fall below the threshold. The system cannot determine a student's intent. An instructor inspecting a student's bubble sheet might also have trouble deciding what the student intended. However, the scanning system ignores any other markings on the form and does not indicate whether a determination was marginal or not.
The threshold can change over time, meaning that it is possible that scanning the same sheet on different occasions might return slightly different results. If bubbles are filled in carefully and completely (and erased thoroughly) then the likelihood of different results is greatly reduced. It is similarly possible that different instructors reviewing the same form might interpret the student's intent differently.
Only forms marked with HB pencil scan reliably. Pens or markers do not scan reliably. Harder pencils do not erase sufficiently and are not always read correctly.