"Your plumbline says my spirit level is wrong".

... and Gary just laughed!  Replacing the porch pillars went really well until the plumbline said we had concreted the metal bases in the wrong place.  However, after agreeing that 12mm in 1200mm was up to our usual tolerance standard, we introduced the new pillars and, behold, they were vertical.  Must be a different gravitational pull on a brass plumbline compared with that on a spirit level's bubble.  I really should have been an engineer...