Hour 0: 
  "I'm gonna finish implementing a feature today."

Hour 1: 
  "Why isn't this working?? This is supposed to work!"

Hour 2: 
  Google, SO, RailsForum, Github, GoogleGroup and even, IRC

Hour 5: 
  "I have tried literally everything. I feel stupid" 
  "..."
  "I might as well try restarting my computer too."

Hour 5: 
  When the restart didn't work
  "Yes, I am stupid."

Hour 6: 
  "Great! I'm stuck at a scenario and a half let alone implementing a feature."

Hour 8: 
  When I eventually found the bug. 
  "WHY AM I SO STUPID! What was I thinking!?"

Moral of the story:

Just because it’s simply one line of code, do not implement unrelated feature B (and forget about it), when you are working on feature A. Especially during TDD.