That sinking feeling you get when you realize the annoying bug you've been working on isn't the cause of your problem.  Or rather, it is, but so are a dozen other annoying bugs, not all of which you know yet, and some of which involve the manufacturing process, and any one of them would be sufficient to cause your problem.

In other words, that sinking feeling you get when you re-discover, for the nth time, that Engineering Is Hard.