If the screener notices it.

One friend of mine passed through security several times (in ROA, EWR, MEM, and ATL) with a large folding knife--probably 4-5" blade--in his laptop bag without anyone finding it. He had the bag hand-searched twice, and the screeners apparently figured they had discovered whatever it was without ever finding the knife. Imagine his surprise when he got home and unpacked and found the knife in his bag!

A couple of vendor reps I worked with on a couple of projects also got away with knives unwittingly. A fellow contractor who is ex-special forces routinely carries several weapons in his carry-on, claiming the ones he has "won't be seen by screeners." (We're talking a small spiked plastic baton, a roll of parachute cord, and some other junk). So far, he's been right--I still run into him at meetings on occasion, and he always produces them at social hours after the meetings.