While mechanical errors didn't prove any more of a problem for procrastinators than their non-procrastinating peers, the same is not true for problems using evidence. Under evidence problems I'm including problems integrating quotations, unsupported claims, and using the wrong kind of evidence (such as quoting others' opinions to stand in as the writer's conclusion). I do not include citation errors. Since these were freshman research papers, most of the evidence was of the sort found in books and websites, but I count examples under evidence as well.
Both classes had procrastinators in the fourth quartile for rate of errors in use of evidence. That's .37 and .13 errors per 100 words for the classes overall but .67 and 1.0 for procrastinators. Looking over the kinds of errors, nothing jumps out as being more of a problem for procrastinators than for the rest of the classes. The distribution of kinds of evidence errors looks to be about the same.
I was a little surprised after finding little difference in terms of length and mechanical errors to find a difference in use of evidence. One explanation that jumps out at me is the possibility that no one--procrastinator or not--is effectively proofreading her paper. I should look back over the questionnaires to see if procrastinators were any more or less likely to report proofreading.
One thing that separates mechanical errors from use of evidence is the expectation of prior knowledge in first year composition. While the classes probably had some instructions in avoiding comma splices, I'm sure if I asked the instructors they'd confirm that more class time was spent on research and quotation integration. So why might non-procrastinators do better here? Well, I'm tempted to think that non-procrastinators were more likely to actually be in class. I don't have any data on attendance, so this is just speculation. But if a student procrastinates because they don't want to work on the assignment, they might just skip class for similar reasons, missing out on important lessons. On the other hand, students with no free time (such as those working full time) may not be keeping up studying or may miss class for other reasons. So I can imagine that students who wait until the last minute to write their papers might also be students who miss out on lessons. But that may be a reach.