Last year I 'smuggled' a rather large pen knife through security at LHR T3 - it was a totally innocent mistake, I totally forgot I had it in my bag and they didn't pick it up on the x-ray machine. After opening my bag in the departure lounge and finding the pen knife, I went back to the security area and very politely informed them about it. I even offered to go back land-side so I could post it home from the mail-box as I didn't want to be found carrying it on me.
Anyway, I ended up being questioned/interrogated/searched for 2 hours and nearly missing my flight... so that's the last time I'll ever go back and point out they're crap at their job...
