I work with adult students (who have to pass an English class before getting to my program classes) and every year I get at least one awful paper that is very difficult to get through reading/ grading.
 
One year, it was an entire paper (printed in color) with the content copied and spliced together from wikipedia and other non-peer reviewed online sources with the links throughout the paper showing up in blue.
 
Usually its just terrible formatting, proofreading, grammar, etc. Like this year, a student used "testes" instead of "tests", so in her paper, diseases can be discovered through diagnostic testes and blood and stool testes. Good for a chuckle, sure, but it makes me sad that you can't even bother to read your own paper before turning it in - or maybe it was intentional to see if I read your paper? I am one of those teachers who reads every single paper.
 
Also, not a paper, but sample resumes, students (yes, plural) spelled my name wrong in the reference section. I've been their teacher and advisor for over nine months and they still don't know my name/ can't bother to look it up online or in the syllabi.