Something isn't working, I play around with the code, and try all possible things in the code. Still it doesn't work. Spend a couple of hours reading each and every line but still in vain. Finally, I find out that I was editing the wrong file (same file at another location) the whole time!! This happened a couple of times when I was a newbie, one of my most annoying mistakes.
Lesson learnt: Now when anybody asks me to debug his code, I first edit/add a print statement to make sure this is the correct file. I thought I was being skeptical, but it has saved me a lot of time (mostly interns do this rookie mistake).