[spoiler] For me it's in Titanicus where Skitaari are attempting to reach a location in a Mechanicus complex. Dialogue runs similiar to this:
Adept 'So how can we get in?'
Skit1 'We could storm the front door guns blazing?'
Adept 'So how far would that get us?'
Skit1 'Approximately 47 metres before we are all killed'
Adept 'But our destination is 800 metres inside'
Skit1 'I see your point'
Adept 'We need to find another way...'
Skit2 'Ooh I've got my grenade launcher with me still!'
Adept 'Brilliant! So re-calculate using the same idea as before including the variable of the grenade launcher, how far would we get?'
Skit1 'Approximately 76 metres before the same fate'
Adept 'Well, top marks for trying boys but I don't think your thinking about the bigger picture here...'
Skitaari look at feet in embarrassment.

I nearly did a little wee the first time I read it, really unexpected like!
So, there's mine, what's yours???