Don't get me started on Architects who know best...
New school built near me. 2 years after it's built the admin offices flood badly during really heavy rain.
On investigation it was found that the single 100mm drain in the flat section of roof above the offices that was put in to collect ALL of the water from the surrounding area (~400sq meters) was blocked by fallen leaves. The water had followed the path of least resistance, as it does, into and down the walls of the offices below.
According the the drawings we had there should have been another external drainpipe running down the outside of the building as a backup. This pipe was conspicuous by its absence. Turns out the architect had removed it as "it spoilt the look of the wall".