Okay, a bit of a rant here - and this does not necessarily apply to any particular airport or airline, but I am constantly amazed how dumb people are when it comes to going through security.

Now, I must admit I don't care to even think how many times I've been through security, but for my own sake (and sanity), and for that of my fellow passengers I have learnt to always err on the side of caution. I always remove any thing that might set off the metal detector or body scanner (including watch, belt, etc) - and prepare things before I get to the trays. A lot of people - including many who seem to be frequent flyers - arrive at the trays in a daze and proceed to unpack their 'oversize' carry-on to find that tube of toothpaste they pack first, only then to proceed through the metal detector and set that off because they forgot to take out their mobile phone, etc.

Two solutions:
i) a 'sin bin' - instead of having one of the security people take time to pat that person down - the passenger is put into a holding area until others in the queue have got through - could be a long wait (although issues with trays backing up...).
ii) better education - since the wait-in line is often long, have more signs/leaflets - maybe even on their printed boarding passes (yes, people don't read them and can be overwhelmed by signs - but there are actually not that many before security). At some places a security person does come along the line (supposedly) checking boarding passes and asking people 'do you have any liquids... mobile phones... laptops...' - which seems to work in getting peoples attention.