Brett,
In my Chinook unit in Vietnam....the entire maintenance crew went along for the first test flight after they worked on the old girls. Funny how things improved after that policy was implemented...funny thing to see the fellows swarming over the aircraft finding rolls of locking wire, the stray tool, and more than a few rags.
At my favorite British helicopter company we had the same policy...and still found rags and tools on the odd occasion. A 212 had a bed sheet sized rag once....and an S-58T had a drive shaft worn all but in half by a rubber covered torch (flashlight).
Thus no one,or no system is immune from those kinds of mistakes. It is the catching of them before something evil happens that makes the difference.