I had grades all over the place, with quarters (I had a quarter system, not semester) where I had a GPA of 4.0 to GPA's of 2.5. My average science class I had a B, not great for applying to med school, not to mention that a B- is worth a 2.7
But overall, I guess I had the worst study skills. And I was over ambitious taking the hardest classes possible (eg. engineering level math and OChem for chem majors) . But one of the biggest mistakes was to be over ambitious, yet not meeting regularly with my academic advisor. I probably made the worst choices during school, taking classes I didn't need to bringing down GPA, not recognizing that I needed help (eg. I should have gone to the Learning Skills Center), and not prioritizing my life.
My epiphany during my undergrad was when I met with my "retention coordinator" (yes, it got that bad) who was a person who I really respected. Basically she told me "You're not studying enough, you need to study at least 3 hours per week for each quarter unit you are enrolled for" (15 quarter units = 45 per week studying). I wanted to get up and hug her. She told me the honest truth. I wasn't working nearly hard enough.
So yup, bad study habits, not fessing up to myself that I was in over my head.