Does the screening really work???

LHR-KUL
Forgot one of my paper cutting blades 5 inches long in my laptopbag. Only realised it once I had got to KUL!!!!! Not noticed at all by the security in LHR.

KUL-LHR
Once I had found out about the balde I palced it in my toletries bag and forget about it until the day of the flight back to LHR. Was stopped and confiscated along with a nail clipper filer (he snaped it of the nail clipper) I had just bought at duty free.