How To Make a Quick Buck
  When you’re looking for a bug it’s terribly hard to not get
  bogged down in details at the expense of seeing the bigger
  picture. An outsider doesn’t have those details so that
  helps tremendously. The lesson I took away from that day is
  that when I’m stuck I ask an outsider.

This pretty much sums it up and isn't limited to software engineering at all. Having chased some hardware bugs on my own I came to the same conclusion: one conversation with a not involved colleague ending in the question Well, have you checked (the obvious) X? was more productive then some hours or days of bug hunting.