Unreadability...
Sometime in the past seven years--since I last graded a huge number of undergraduate papers (for what are teaching fellows for, after all?)--my handwriting has deteriorated from nearly unreadable to completely unreadable.
Memo to self: in future grade undergraduate papers in block print with Sharpie (or make students submit pdfs and write notes on them in Skim http://skim-app.sourceforge.net/).
