The use of Ink pen and Gel pen may cause smudging or blotting, so avoid using this type of Pen. Use Gel pen only if the ink dries out immediately. If the ink does not dry immediately, then the ink may spread over the adjacent bubbles.
In case of duplex or two sided sheets, if we use ink or gel pen, then there is a possibility of impressions on the other sides of the sheet. The impression marked by ink pen or gel pen does not show up immediately on the other side. And based on this if you go on marking with the same pen, gradually after some time, the marks will start appearing on the other side. Then you cannot do anything. This will ruin your sheet.
Sometimes candidate use the proper pen at the start but later due to defect in ball pen they start using gel pen or ink pen and later they discover that the new pen is creating impressions or smudging.