Speaking as one who fell for it before, BA doesn't think that their staff are stupid enough to fall for it. They KNOW the staff will fall for it; they have pulled this stunt several times in the past and got away with it. Once I realised what was going on I quit and got a real job!
We engineers are a such a bunch of trustworthy, reliable and conscientious gentlemen (see courtesy thread) that we don't believe that others (in the anti-personnel department for example) are such a bunch of thieving, lying, scheming b*st*rds as to do such things to other people on purpose.
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Through difficulties to the cinema