Everyone knows the corny scenario where a hobbyist builds a magnificent model in the front room - only to find that they have to knock down the wall to get it out.
On the giant scale of the main terminal at Heathrow, the possibilities for boxing-in disasters are multiplied a millionfold, and the potential cost and time overruns as well.
The interactions between the many jobs, and simultaneously operating skills and trades, make it so much more complex.
Spotting these unforeseen problems from the definition stage onwards and organising construction sequences and timings that avoid them, is at the core of the job carried out by the T5A production integration team led by Mace, working with BAA's strategic project management.
The production integration team is responsible from the moment the work actually begins for production leadership and integration.
Planning, co-ordination and monitoring of all the work is in its hands. Controlling costs, minimising waste and avoiding site delays are crucial.
'This means identifying all the scope gaps are covered, ' says Gareth Lewis, the team's production leader, 'spotting those things which are not in the architect's or engineer's designs, so that you can optimise sequences of supply and delivery.'
Communication is vital, he adds, because 'the biggest challenge is to ensure everyone understands sequences and the next phase of the task'. To help with this, his team is integrated with the overall BAA supply organisation, making up about 10% of the 500-strong team.
'The matrix management structure of the project, which allows communication project by project or by system such as quality control or cost control, has proved absolutely essential, ' he says.