Then again, how many times have you found a bug caused by a single line or function call or bit of syntax that didn't do exactly what you thought, that was easily overlooked? Particularly in someone else's code in a language that isn't your main one. I think line level interactive debugging is valuable precisely because it tells you what you really know for certain at the local level, and so lets you reason more effectively about the big picture.
You can discern those through thinking, too. "Well, it got to this point and broke... hm... well, what do I know about this environment at this point. What assumptions am I making? Since this assumption isn't being fulfilled, it must be this issue... and that was caused here"
Also, this was pair-programming, both people are making sure that what's written is what's intended to be written and so those issues should be (mostly) gone.
