Taking out student loans to go for a master's degree when I should've just stayed as an undergrad a little longer and switched majors/double majored in CS. It made sense to me at the time, but in retrospect, it would've been less of a hassle to just stay an undergrad for another year and a half and just got my bachelors in CS. It wouldn't have been so bad except I ended up flunking out of grad school because of a combination of personal issues leading to me not doing well in classes and a school administration that really screwed me over in my last semester.
The only bright spots are I didn't have any debt from undergrad (so I'm not super deep in debt with my loans) and the field I'm trying to get into now, software development, isn't one where a degree in a specific subject is a hard requirement everwhere. Still, not having a CS or related degree is definitely making it harder to get my foot in the door.