A lot of the students write their homework assignments on cheap, pulpy, absorbent notebook paper that is only a step up from toilet paper. I'm using my usual gel pen for grading, and it soaks right through the worst of the papers. This is worse than an annoyance, because most of the assignments are written on both sides of the sheet. The ink wicks through and obliterates parts of the teeny, faint pencil marks that the student scrawled on the back of the sheet.
So tomorrow I'm going to buy colored pencils for grading. It's much easier for me to adapt than to convince 113 students to use decent paper.