Not starting two assignments that were due in on the same day until the day before. I ended up completing one to reasonably decent standard, and handing in the other half finished (1500 words below the word count) and without a bibliography.
Got 30% or something like that for that second one, which was 50% of the module. I was angry at myself as I was on for a comfortable 1st for both that module and my first year, but dragged the module grade down to 50 something %. Luckily, I still scraped a 1st for the year with a 71%.
It was only first year, so on the whole it doesn't matter, but knowing how I felt afterwards I think will stop me making the same mistake again. I remember feeling so guilty and disappointed in myself for getting lazy on one of my final assignments after working pretty hard for the entire year up until that point. Especially when I thought I might not get the 1st that previously looked nailed on.
It's sort of a non-problem now since I guess things worked out, but at the time I wasn't happy with myself.