...this can only end in a horrible work/dis/me getting sacked mix up can't it?

Also: what's the correct etiquette when someone in front of you holds open a whole series of doors for you?

a) Thank them each time
b) 'Thanks' for door one, 'Cheers' for door two, Smile only for door three, weary nod for door four (my chosen option)
c) Kiss?
d) Other